GTA
Airguns by Make and Model => Benjamin Airguns => Topic started by: jonhunt82 on January 24, 2015, 08:31:30 PM
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I read that break barrels do no accelerate the pellet through the entire length of the barrel. I called Crosman and found out i could get a new barrel for $20.. So the fun began
After polishing the air chamber and other rough spots in the power plant, i changed the seal and moly pasted it.
Chrony with 19.5" barrel, 680 fps (+/-10) at muzzle with JSB 25.39gr
Using a 2pc 9/16 steel (hard) shaft collar as a guide i cut the barrel with a hacksaw. De-burred.
Chrony with 15" barrel, 715 fps (+/- 7)
at this point the barrel as a cocking lever was a little hard on the hands. so i got a little tacticool. I cut a flashlight handle (1"id) and attached it to the barrel. I used 2 of the (9/16id x 1-5/16od) shaft collars 2pc clamp style on both ends of the flashlight handle. Looked like a can. But it made it alot easier to cock. I slid the whole "can" down towards the breach and found that i could cock the gun alot easier with the wide grip. SO back to cutting.
the overall length of the "can" was 6 1/8" i randomly cut it so i wouldn't have the flashlight button showing so there was no real science to to the OAL. I went for broke I cut the barrel so that the "can" would be flush with the breach block.
Chrony with 10 1/4" barrel, 666fps
looks like i missed the window, next barrel is on order. going for 12" I hope.
I recrowned the barrel.
Now for the accuracy and the "cure-all" section
the Barrel has a loose bore, i already lapped the breach side with lapping compound. I noticed that the shaft collars can add choke, a noticeable one, so much that i am worried about ovaling. Alot easier than the other methods that i have read about. But the one difference is you have to leave the shaft collar on the gun. I only had a 30ft range, but i was 4" low same hole. Thats the best this gun has ever shot. I checked my scope and i was too near the end of my elevation adjustments at 30ft. So I had a droop, or the bore wasn't straight/dead center. I raised the poi 6" by shimming the "can". I put a random piece of Al sheet between the breach and the first shaft collar.
So I am pretty sure my tacticool Can has an adjustable choke and bent barrel compensator. Jackpot
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some pics
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Made a pumper with a NP725 barrel got 630fps with 20g FTT still have the barrel and a much larger valve should be able to hit 700-725 fps.
on my pumper it was an accurate barrel.
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Interesting. Must be a bear to cock that rifle.
Scotty
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It isn't easy to cock it takes a little more method than strength. Some people try to scissor them, I just place the butt of the gun against my abs/ hip area and make a motion like a one handed pullup. This method makes cocking a magnum springer easy. But I do have some concerns about the cocking effort that might dissuade some people from cutting their barrel. If i were to have a beartrap issue, i am not confident that i can hold the short barrel safely with my left hand. all 10 fingers crossed :-\
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You are almost making me want to cut mine, we will see with the next design. When's all the stuff get here?
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called Crosman today and they said it shipped and will be here by Friday . I am way too paranoid that i buried it with the snow blower. Metal is here, folding stock adapter is here, next week ill have, AR buffer tube, 6 position stock, and 2 hatsan airstrippers 1 in .25 and 1 in .22 (Midwayusa clearance). Don't tell the wife. The stock is wicked siiiick. (http://www.tickbitesupply.com/stk_tbtar2m_d.jpg) (http://www.tickbitesupply.com/stk_tbtar2m.jpg) (http://ecx.images-amazon.com/images/I/410tQPNWpPL._SL500_AA300_.jpg)
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Historically speaking, it has been determined that 'springers' do better with the mid/standard weight, for that caliber, pellets. Tuners also figured out years ago that barrel lengths around 9 inches are needed to fully accelerate the correct weight pellets using the short duration/pulse of compressed air generated by this type of powerplant. So they added external shrouds or brakes to increase their length, reducing the cocking effort.
http://www.arld1.com/pistonpelletdynamics.html (http://www.arld1.com/pistonpelletdynamics.html)
Compressed gas types (pumpers, Co2, PCP) on the other hand, need the extra barrel length for the volume of expanding compressed gas released to accelerate the pellet.
There is a balancing act that is specific to each type of power plant.
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Good simulation of springer dynamics.
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I'm wondering where and who determend years ago that 9 inches is enough . I say that because I have done some barrel chopping myself . From what I read the tx200 has 9". Early testing showed that is enough. But they weren't testing springers with 25 ftlb and up power plants. Seems to me they need a little more barrel on the benji xl or hatsan 125,135 or they lose fps. Dont have any hard numbers myself but I think for the larger springers its probably closer to 11 or 12 inches.Kinda seems like your numbers support that . Mine do.
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Great find Yrok, that picture is really worth a 1000 words. I wish i had that when i was explaining this to my friends. The science behind this is mind boggling the graphic did not cover half of it. The most interesting part to me is the fact it isn't the change in volumes that are the sole factor in propelling the pellet. The volume change increases pressure rapidly, creating alot of heat expanding the air more-so but that is also followed by a rapid cooling. SCIENCE! lol ;D
BobbyHumphrey : there are tons of factors in barrel length. The powerplant has to be in tune with the barrel length and tightness of bore. Less resistance in the barrel means that the high pressures will persist farther down the barrel. Id guess the mass of the pellet might help initially but i think friction wins after a couple inches.
It leaves me with a headscratcher... With a magnum springer we might conclude that a longer than 9" barrel is needed. With the increased volume of air and longer stroke is the pellet moving at the initial lower pressures? So would that mean a tighter breach would be beneficial with a shorter barrel?
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Hi Jonhunt. I saw that graph as well while researching barrel length. I read so many blogs and articles .I'm not sure how I came about my conclusion that for a magnum you do need more than 9 inches. The science is beyond me. What you were saying about expanding air and restricted barrel and heat I cant comprehend in terms of barrel length in exact numbers . But I did chop my Walther Falcon Hunter to 9 inches [ its a re branded Hatsan 125]. Well really my dad chopped it first to around 9.75 and he had a chrony and he said it lost some fps. I chopped it some more to re-crown it and allow me to mount a shroud the way I wanted. But the gun is still powerful . And I would say less hold sensative. Basically it is my favorite break barrel. And its 32 inches! That is with the shroud which completely does its job. Because it was loud! Much louder than my Benji Super Streak.
Now since you have a .25 cal the dynamics are probably different . I think its awesome that you chopped yours and are documenting the results here. To be honest if I could afford a .25 cal barrel or another .22 [ Hatsan replacement barrels are twice as much as Crosman and you gotta pay them to put it on ] I would try 12 inches .
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Bobby: I got a pm by another member asking if the shorter barrel made the gun louder. I personally didn't notice any difference between 19.5 and 15.. I think the 10 1/4 is a little louder, but not enough to complain about. Makes sense though if the sound is created by high pressure air leaving the barrel. Did you notice increased loudness on your falcon?
I don't have the equipment for this but this brings me to an idea. It might be possible to cut a barrel to near perfect length if you use a decibel meter. If it is true that there is an increased loudness due to high pressure air. I think ill stick to the chrony cut chrony cut method.
I will be doing a full barrel workup with my previously mentioned method and i will post it all here at GTA. It will be on this thread unless i make a full guide and then ill put a link to it here in this thread. Still waiting on the mailman.
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Yes , it is much louder. The walther and benji were close before dad cut the walther. I hope I didnt lose too much power but like I said it seems pretty strong. If anyone has a 25 cal barrel I'd love to start over after you determine a good length.
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UPS just dropped off my barrel assembly , looks nice, straight, no mars or blemishes. I haven't tried the fit yet or checked the bore beyond a visual inspection.
tip #2 for all that might try "my" mod, Buy a 2nd barrel first if it is a newer gun. The thought never crossed my mind until i got the new barrel assembly... On the trail the model and serial number is on the breach block which is part of the barrel assembly. New one is blank. So if you were to have a warranty issue it might be best to keep the original barrel that came with the gun untouched. Crosman likes to send you a "new" whole gun replacement on warranty issues, so save all the stock parts you swap out as well.
I can't wait to get this started.
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That's a good tip about the barrel. You never know if you'll need the warranty.
As far as the barrel lockup, what did you find out. My question mark key doesn't work so..Remember, no barrel play and no rubber washers! That can't be good! I even shimmed my pivot bolt with a 1.5 thousands feeler gauge to take out slop I had there.
Also , I don't know if this will help but I have chopped and re-crowned many barrels and then shot to see what happened. What I have found to work for me is some things I picked up on GTA. I cut the barrel and file it as perfectly square as I can, I take the time to make sure it is very square. Then I use a flat 2 sided stone my millwright dad left. I don't know whatit's properly called. I work it as smooth as silk. Then I use a brass screw, and car polish, that is about a sixteenth bigger than the barrel, I go forward then reverse ending with a very light touch. I finish with a soft dremmel stone by hand ,checking it with a magnifying glass and q tip. I'm getting three quarter inch groups at 38 yards with a beeman rs2 where before I was getting 1.5 to 2 inch. And that's with gammo hunter pellets which may not be the best for this gun,
I'm a student who actually is running in the negative right now so I can't afford much so to fix the trigger I used a bike chain bearing for the washer trigger mod. Removed that spring,cleaned and slick 50'd it and I really like it now. Other than not being 2 stage I must say its perfect. I also cut the trigger a little shorter , removing some of the curve and that help my pull and feel.
I know you may already know this or perhaps are a machinist with access to to an ultimate set of tools but maybe not or this will help someone else who is like me and only has a drill and hand tools. So forgive me if I sound like a know it all because I don't mean to come off like that! Rock on my friend.
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I pick up what your putting down bobby. "my machinist" isn't readily available to do any R&D. He will help me if I already have a sound plan in place. I am a firm believer in use what you can, make it work, and as long as you made an improvement you did just fine. If I had a lathe and a mill trust me I wouldn't be toying with mods, I would be building as much as I could from scratch. I have yet to see a spring gun design with the main tube recessed deeply in the stock, that would shorten the gun about 14". Or a design that allows the scope to be mounted on the breach block or barrel. But I am derailing my own topic. Getting back... I might pm you bobby if I have issues with loco. The only reason that I haven't tortured it, is because my machinist wants to see if he can get the whole barrel assembly in the lathe balanced. I might be attaching a hatsan.25 airstripper. And there is only a few ways to attach that properly..
Posting on mobile isn't easy... Loco = lockup, torture = tested
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Cool. You threw me when you said loco. I was wondering how I could help you with that! but yeah I reworked the detent , shimmed the spring, and polished the whole thing. It was killing me trying to tighten my group and I could feel vertical movement when it was closed.
I looked into that hatsan air stripper. Nice. I made my own out of a fuji water bottle top. The kind you pull to open, it looks like a cone on top of a regular cap. It fits inside the inch and a quarter shroud with no lateral movement. Same shape as what I believe is ideal and the hole was perfect. Took some time to get the correct distance from the end of the barrel but I started with 1.5 times pellet width. I can adjust the distance by changing a spacer.Air diverts back into shroud. Eight evenly spaced holes near the breach end of the shroud for air pressure relief.
I read a long post by Bob Sterne [or was it Nervousetrigger] I believe and he was saying all you need is a 90 degree cut and smooth lands for the crown. The method I described got that for me. Shroud protects crown so no need to recess.
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Where I said detent I meant alive jam. I always mix up those terms.
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so i started work on the new barrel.. I took a bunch of pictures of the process. I may have rushed it a bit.
i took 3 shots through the chrony with the full sized barrel for a bench mark, then i cut and de-burred the muzzle. I took quite a few shots noticing a wild deviation. At one point i broke 900, which is near impossible. So it must be the lights, i had 2 weak led touch lights as backup lighting, and it could have had issues with the sunlight coming through my basement window. I know to keep the florescents off during testing. So with a 150watt single bulb i got a whole new set of numbers, more consistent but low.
I then ran a pellet through with a cleaning rod, very tight breach and very loose bore after. So i lapped just the breach, then cleaned. This bore was much looser than the original and i haven't measured it yet but it looks off center as well.
After getting an average at 14" i then cut to 12". It takes 1-1/2 hacksaw blades per cut. Then i recrowned the barrel and added my choke collars at the muzzle. The avg fps for the 12" is around 680 with a 25.39gr jsb. The barrel might break in a little more and give me a few more fps.
I based this entire project on a previous barrel, i had numbers for 19.5" 15"and 10.25" . Now the numbers.... I was unaware of the 60hz issue with florescent lights, the 19.5" and the 15" were both done in a room lit by florescent lights with a shop halogen hung directly over the chrony. The 10.25" barrel was tested with the same chrony but only in incandescent lighting. So i am not sure if i trust my numbers completely.
moral of the story thus far.. Don't compare one barrel to another barrel, use proper lighting when using a chrony, and most cheap barrel manufacturers only care if the bore is centered at the breach and muzzle, so when you cut it there may be centering issues.
So whats next for this project.. I am going to get more numbers. Take the gun apart check seals and relube. get more numbers. I then will compare the 10.25" barrel and the 12" barrel and fit the more accurate of the 2. I might be unable to fit the hatsan airstripper to either of the barrels due to the misalignment of the bore. Then I'll send it to my machinist and have the cocking "can" put on. I should have more to add by the end of the week.
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Manual barrel cutting... It took almost 2 blades to make each cut. I used 2 9/16 (barrel diameter) washers and shaft collars to square the cut. Place a pellet in the bore to stop the bulk of oil and metal filings from getting into the barrel.. Oil well.
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A few comments. First, thanks for the message reply. Have not answered because nothing to add. Second, thanks for posting the pic...good way for us home machinists to cut straight. And the washer/collar could be left on during the polishing of the muzzle face, yes?
Third, i've rolled two different oem barrels for my NP on my granite machinists flat and both were curved out of the package from Crosman. So obviously the barrels are not precision parts...wouldn't expect that at these prices. And because the outer surface is curved, there is no guarantee the bore is straight. But the curved barrel "could" have a straight bore. And after straightening the barrel so that it rolls flat, one could unknowingly wind up with a crooked bore. It's a crapshoot. HOWEVER, there is a famous picture out there of a airgun barrel with a loop (at least one loop, maybe more than one loop, i can't remember)...but the last few inches of that barrel is straight...and the claim is the gun is accurate. This would indicate that worrying about barrel straightness, bore straightness, or about whether the bore is coaxial with its barrel...is pointless. If the last few inches is straight, you're good.
Fourth, chrony velocities will be weird if the lighting is not proper and consistent...that's for sure.
Have fun.
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i have seen that old school twisted barrel photo. what a trip. The first barrel i crowned I left the collars on. They were harder steel than the barrel so i thought it would be a great guide. This trick with the washers/collars made the face pretty flat and i didn't have to remove much material at all. I used a bastahd file for a few strokes to eliminate the saw scratches, then i switched to course diamond, fine diamond, then ark stone.
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Mod still in progress. Waiting to sit down with my machinist ...meanwhile...
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I flipped the folding stock so it opens to the right rather than the left. I also gave it a coat of paint, then liquid rubber, then i textured it, then sealed it with more flat paint. I also relocated the sling stud to the pistol grip.
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I flipped the folding stock so it opens to the right rather than the left. I also gave it a coat of paint, then liquid rubber, then i textured it, then sealed it with more flat paint. I also relocated the sling stud to the pistol grip.
Thanks for the ongoing pic's but we gotta pass the hat and get you a blue screen and back light. ;D How about a pic (bird's eye view?) of the gun with the folding stock...unfolded - because i know nothing about tactical type stuff and so no idea what you mean...did you flip some hinges?...and why does the stock fold anyway? And i forgot...what are you planning to use this gun for? Is that liquid rubber like the stuff you dip metal tool handles in to make them more grippable?
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john,
I am just doing an interest piece here. I plan in the future to make a step by step guide on how to make the mod. So far the topic hasn't been very popular so to avoid reposting everything in a more organized format i have been a little quiet on the "hows and whats". But i am happy to answer any questions.
The pics are crappy because i have a data limit and i can't take them with my phone, so i have been doing it with a tablet. As for why am i doing this to the gun. I take it with me backpacking/hiking, i plink and hunt. People of Massachusetts are terrified of guns and a friend of mine lost his real gun permit by looking at a pellet gun for sale in a public parking lot. People see a gun, assume it is a firearm, assume that you are up to no good, call the cops. I have had the police at my house 3 times for backyard plinking. So my Mod shrinks the overall size of the gun to make it more concealable so it doesn't look like a real rifle. OAL right now folded is 27 1/2" and if i cock the barrel 20" a big improvement from the original OAL of 48 1/4". The Liquid rubber is something like "flex seal" it is a rubberized coating without asphalt. It is called "leak seal" by rustoleum. I have called rustoleum and I couldn't get any useful information out of them, and they won't answer any off label questions at all. So i am kinda testing it myself. The stuff is self solving, and can be thinned with acetone, it is best painted on with a foam brush and as it dries you can texture it with the same foam brush by blotting it. It takes forever to cure, so thin coats are best. I personally like to put it in my oven at 175deg for an hour to speed the set time. Once fully cured (a couple days id say) spray paint adheres well to the leak seal, use very thin coats, and allow a full cure on the first coat. Spray paint will solve the leak seal, spray on too heavy and it will "melt" away your texture. I like to use a heatgun to warm the stock before spraying on the first coat of paint, this helps the dry time.
Ill post a few more pictures.
thanks for the interest
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the floor tiles are 9"x 9". The barrel work isn't finished yet
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Good job . I like what you're doing . If you want to put it in a backpack without cocking it maybe you could put something over the barrel . Someone here may have an idea or two if your not sure. Like the end of a baseball bat so it looks like a bat. Even a cheap plastic one.
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LOL bobby, That would be great.. put some eyes and a reel on it.. I do have a trick up my sleeve that might be Horrible to post here.. (admin please delete this post if i cross a line)
i have considered painting the tip of my muzzle orange, that might throw some people off my trail and maybe ease the police response.
My legal hunting area is shared by hikers and bikers. Some of them are vegan tree hugging animal rights activists others are not. During hunting season which is almost all year round it is legal to hunt sunrise to sun set for the majority of animals (some can be hunted at night, coyote, raccoon....). I have heard quite a few stories of hikers walking towards a drawn bow. It isn't the hunters fault that the hiker didn't respect hunting season, and i bet you the hiker would have given the Arch'er (no idea why thats a swear) an ear full if he wasn't full camo in a tree stand. I have to move out of MA.
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Diggin it, very cool modding!
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This topic started as a barrel mod, and i have been trying to come up with some ideas for the mod. So in a nutshell it is a cocking handle for a shortened barrel that chokes the barrel, allows for a non-threaded airstripper to be attached, a reduction in noise by design, and the ability to straighten a curved/drooped barrel.
I have everything going smoothly except for the "straitening of the curved/drooped barrel". My original design allowed this adjustment to be made with shims and it worked great. This method might be the only/easy way to go. I was thinking of ways of using adjustment screws to reach this goal but i can't figure out a way to adjust all 360deg of the barrel. This screw idea would allow windage adjustment L/R but no elevation U/D unless this device is welded to the barrel.
You might ask. "Why bother besides fixing barrel droop and staying within the scopes adjustments?" Well if a barrel can be shimmed or adjusted, a 2nd barrel can be adjusted with the same method. This would mean an easy barrel swap with only a vertical poi adjustment.
also I had another thought about barrel choking. My shaft collar method only chokes the required 1" at the end of the muzzle. While I was working on the gun yesterday I had the crazy idea that if you completely covered the barrel in shaft collars you could make a tapered choke by using a torque wrench. More torque at the muzzle and decreasing towards the breach. It would be a little pricy at $10 per inch.
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I just put an aftermarket trigger in the gun, what a huge improvement over factory. You might be looking to do the same with your own trail, and i do suggest it as a custom improvement but i still stand by my opinion that the trigger itself shouldn't be a huge accuracy improvement if you know how to shoot a springer. Also i have used 2 different aftermarket triggers in the trail, both triggers could be adjusted enough to suit my needs. Both triggers were gold.. The one I personally installed was purchased with a credit card and shipped from Arizona. The main reason i went with the unmentionable trigger that isn't advertized much here is because i could use a credit card to buy it. Someone asks for a money order, i only do business with them as a last resort.
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more progress. The cocking handle has an adjustable airstripper built in. It can be tuned with an hex wrench much like the hatsan design.
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I am still not done with the project. After making it all pretty and getting some very nice groups @20 yards i have noticed a considerable barrel droop. I am maxed out with scope adjustments and i am still shooting 5 inches low at 20yds. I tried to work with the breach seal and washers to raise my poi. I could shim it like I did with the last design but i have put too much work into this for something that simple. I am going to add a threaded rod to the mod that will turn into a adjustable droop compensator. Once i get that done Ill post pictures of the internals of my cocking handle.
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bull pup springer...look up qb57
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Qb57, I don't even consider that an airrifle because I can blow a pellet out of a straw faster than that gun can shoot. It has low pistol velocities, neat concept though
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Hey it'll stick with or beat most of the sub $50 pumpers