Trail NP XL .725 Tacticool mistake turned cure-all

[jonhunt82]

 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 

[jonhunt82]

some pics

[K.O.]

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.

[LAalex]

Interesting.  Must be a bear to cock that rifle.

Scotty

[jonhunt82]

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 

[neonmoon]

You are almost making me want to cut mine, we will see with the next design.  When's all the stuff get here?

[jonhunt82]

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.

[anuthabubba]

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

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.

[DOKF]

Good simulation of springer dynamics.

[BobbyHumphrey]

   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.

[jonhunt82]

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

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?   

[BobbyHumphrey]

   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 .

[jonhunt82]

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.

[BobbyHumphrey]

 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.

[jonhunt82]

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.

[BobbyHumphrey]

   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.

[jonhunt82]

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

[BobbyHumphrey]

   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.

[BobbyHumphrey]

   Where I said detent I meant alive jam. I always mix up those terms.

[jonhunt82]

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.