A question about exhaust port v. transfer/barrel port size

[HillGSA]

Transfer sleeve may not be sealing properly/consistently, during the shot.

[Bullfrog]

I have checked the breech oring, it is still there.

I would suspect that if my transfer ports aren't sealing properly I would be experiencing unstable velocities, correct? Yet Saturday when I had the gun on the chrony I had nothing but stable velocities for all of my strings.

This evening I'll patch it out to clear and then put a scrub on it to see if it knocks anything significant loose. If not, I'm back to the drawing board.

[t_jones98]

Initial thought would be to the most invasive step that you did with the last mods: drilling the port.   My hunch is that a burr has been left which is throwing off a skirt.   

Load a pellet as you would with the mag (I'm assuming that you're using one based on your comments), but then disassemble enough to push the pellet through with a dowel. 

[Bullfrog]

Initial thought would be to the most invasive step that you did with the last mods: drilling the port.   My hunch is that a burr has been left which is throwing off a skirt.   

Load a pellet as you would with the mag (I'm assuming that you're using one based on your comments), but then disassemble enough to push the pellet through with a dowel.

That's what I've been scared of. However, let me update ya'll on where I'm at. After several hours of working on the gun last night I'm only a little bit closer to getting is figured out, but I think after this afternoon I'll be able to eliminate some possibilities.

Yesterday afternoon I took the barrel off the breech and started the same cleaning regime I did 2 months ago to free it of the terrible fouling it had. The only difference is this time I used 4 fresh brushes instead of the worn brush I used then. I started with Shooter's Choice lead solvent, CLP, and SC's Quickscrub III. I then brushed out until nothing else came out. I then used a brush of JB Bore Paste and scrubbed it through good. I then spent the next 4 hours scrubbing and patching. When I finished I was still getting soot out after a brushing but the bore visibly looked shiny and I decided that surely all that must be left was light residue that kept getting scraped out. That, or my brush was actually putting it back down, as there seemed to be no end to the soot.

Around midnight I set up at 20 yards and shot 2 four shot groups with Barracudas. Beautiful sub-dime groups. I then refilled and loaded up with 4 Barracudas and 4 Eunjins and went to 30 yards. Those groups were terrible just like all the others.

So 1) I may still have a barrel problem, or 2) the big, quick, blast of air is somehow destabilizing the pellet. I'm sure there are other possibilities, but one of those two are where my suspicions lay. I need to shoot more groups at 20 and then 30 yards to see if the pattern stays consistent where 20 yards is tight and 30 is all over. That would tell us something.

As hard as I studied the barrel last night I don't see where there are any jaggies at the barrel port.

I have one solution that may help me eliminate or confirm the barrel as the problem. I have another barrel here that is brand new that I can try. If I drill out the barrel port to the same size and I'm careful to not leave any jaggies that should help me at least rule out my barrel. I really don't trust my barrel. I've road that poor barrel hard for 2 years, 10,000+ pellets, and speeds it was never meant to see. If it was a horse, it would have been shot by now to put it out of its misery.

If the new barrel performs the same (assuming no jaggies), and if a pattern emerges of tight groups up close and wide groups after a certain point, wouldn't that suggest the problem is with the movement of air itself? Perhaps destabilizing the pellet? If so, I would presume my options would be to either choke the port back down or try bullets. Thoughts?

I'm done with work obligations today. I've got plenty of daylight to work on it. I'm getting on it now. 

[wwonka]

If the new barrel performs the same (assuming no jaggies), and if a pattern emerges of tight groups up close and wide groups after a certain point, wouldn't that suggest the problem is with the movement of air itself?

Interesting thread. We airgunners become gunsmiths and ballistic detectives even more so than PB shooters...

One possibility (of several) is that you have so much energy now that within a close distance the pellets are moving supersonic or near supersonic - and as they decelerate rapidly (which diabolos do) - they hit the trans-sonic chaotic zone and destabilise.

Another possibility is that the pellets are already destabilised by something right from the muzzle (super blast of air, burr in the loading area) - but the effects naturally become magnified and noticeable at longer range, and as the pellet slows down and loses whatever early aerodynamic stability that it had in its flight.

[Bullfrog]

I think I've found the likely culprit. The only question now is going to be figuring out how things got to this point.

I took my gun out to 20 yards with the chrony and shot 3 groups. 1 at max pretension, 1 at min pretension, and another at min pretention with different pellets. First, my numbers:

Group 1
Kodiak Match (Barracudas for all practical purposes)
Max Pre-tension. 3200psi fill
990.3
988.8
1005
1006
1032
1021
1019
 
Group 2
Kodiak Match
Min. Pre-tension. 3200psi fill.
709.5
696.1
686.6
679.8
695.7
691.7
710.0

Group 3
JSB Kings
Min Pre-tension. Continued from Group 2's fill
724.5
723.4
731.1
741.0
742.2
739.4
744.5

As you can see, all velocities for each group were consistent enough to test accuracy. All of the groups were atrocious. The worst was the Kings. The most interesting observation was the massive POI shift between the high and low speed groups of the Barracudas/Kodiaks. I marked the point of aim on each target.



So my observations from last night were flukes. My groups are off whether I'm up close or far away. Speed itself is not the issue, neither is inconsistent velocities.

t-jones gave me a good idea. Push a pellet through the barrel. I did so and found no nicks. Unfortunately, I also found barely any rifling. Bingo. I then pushed a pellet through the new barrel. The difference is night and day. Take a look:

 

Pellet on the top went through the new barrel. Pellet on the bottom went through the old. That's got to be the problem. The old barrel had a rough spot in the middle but otherwise the pellet met no resistance through the barrel. The new barrel gave the pellet no resistance until the end, where it is apparently choked or the rifling is deeper and it took some effort to push the pellet through.

Question is, how did this happen? Did my use of JB on a brush do this? Or have I just worn the barrel out? I posted a thread about barrel wear back a few months ago and it was generally agreed that an airgun barrel can't be worn out through lots of shooting. Therefore conventional wisdom would say it was the cleaning. Yet, I did have this same accuracy problem before I even did any special cleaning except run patches through it (and a brush a few times) over the past 2 years. About the middle of last year I started getting spotty accuracy. Superficial cleanings would solve for about a month and the problems would return usually worse. Finally the aggressive cleaning I did 2 months ago restored superb accuracy until this past week. Had I worn the barrel to almost nothing in December and the deep cleaning just prolong its life? Or was it severely fouled in December and my aggressive cleaning remove the fouling and then the rifling too? I can't say. I'm not sure there's a way to know or what lesson there is to learn from this.

I wonder if some pellets are too hard? I love the big Eunjins, yet I've heard it said they are made of very hard lead (I always thought they were soft because of the way they expand at velocities above 750fps).

[wwonka]

Just to be completely sure it's not some random effect (like the close-range/long-range grouping "differences") - have you tried pushing maybe 3-5 pellets randomly chosen (shuffled or shaken up) through each barrel and comparing the rifling?

This would eliminate the possibility that one pellet had a different diameter from another. I couldn't tell from your post whether you've already done this, but if not it might be worth it before doing major barrel work.

[Big Bore Bart]

  You say you used brushes for cleaning, what kind of brushes?    

[Bullfrog]

Bronze. I basically followed the advice here: https://www.pyramydair.com/blog/2005/11/is-your-airgun-barrel-really-clean/ The only difference is I tried hard-core lead solvents first like Shooter's Choice and traditional solvents like Hoppes before I was brave enough to try JB.

For 1 1/2 years I never did anything to my gun but run patches through with a mild oil or cleaning pellets. About the middle of last year my accuracy went away and wouldn't be restored with any amount of patching. I lightly used a bronze brush and that restored my accuracy for a few weeks, but by late last year even a light brushing wouldn't do it. I then started using FS solvents and that helped for a little while. I was convinced that the problem was not using a strong enough solvent to cut the lead residue. CLP really seemed to do the trick for a month or two. Then by about December of last year nothing would restore accuracy. Someone recommended JB to me and it worked like a charm and my gun shot better than it ever did from the beginning of January to last week. During that time I patched it out twice but did no brush work.

For about 6-8 months there's been a gradual decline in my barrel's health that traditional AG light cleaning stopped fixing. I can't remember any variable that began about that time, except me hot-rodding the gun for an extra 100fps. Although I'm suspicious of the Eunjins in fairness to them I didn't start using them with the power tune until January. Previous to that, my power tune wouldn't shoot them well. I thought it was the speed until I gave the gun the JB cleaning in January and then they grouped great until last week.

[Bullfrog]

Here's my old thread where I was talking about my accuracy problems back in December.

http://www.gatewaytoairguns.org/GTA/index.php?topic=58428.msg565321#msg565321

[Big Bore Bart]

  OK here's my .02c.     Bronze brushes + slightly corrosive solvents = dead barrel.   Air gun barrels are not much, if any harder than the bronze alloy in the brushes.   Brass brushes are softer, but not by much.      Shooters Choice is corrosive, as it has to be to dissolve copper and lead fouling.   The so called "soot" is actually Iron compounds.     One test of the barrel with the bad spot in the mid section, to see if it is just still fouled, would be to plug the muzzle and fill it with S/C lead remover and let it set over night.  After all if it has no rifling left, what will it hurt.

[wwonka]

... out of abundance of caution I don't use any bronze or brass brushes on my airgun barrels - just an occasional cleaning snake, cleaning patches and ballistol or kroil...

[Bullfrog]

Just to be completely sure it's not some random effect (like the close-range/long-range grouping "differences") - have you tried pushing maybe 3-5 pellets randomly chosen (shuffled or shaken up) through each barrel and comparing the rifling?

This would eliminate the possibility that one pellet had a different diameter from another. I couldn't tell from your post whether you've already done this, but if not it might be worth it before doing major barrel work.

I've tried some different pellets. The results are consistent, the new bore definitely leaves more defined rifling than the old. However, I found that the old bore will leave rifling on JSB Kings, albeit still not as defined as from the new bore. Interestingly the Kings in both bores receive rifling on the head as well as the skirt. Also interesting is that in my old barrel Kings shot the worst today.

OK here's my .02c.     Bronze brushes + slightly corrosive solvents = dead barrel.   Air gun barrels are not much, if any harder than the bronze alloy in the brushes.   Brass brushes are softer, but not by much.      Shooters Choice is corrosive, as it has to be to dissolve copper and lead fouling.   The so called "soot" is actually Iron compounds.     One test of the barrel with the bad spot in the mid section, to see if it is just still fouled, would be to plug the muzzle and fill it with S/C lead remover and let it set over night.  After all if it has no rifling left, what will it hurt.

You might be 100% right. At this stage I'm perplexed. I had another pair of eyes look at both bores tonight while I was in the shop. His opinion was that the old bore's rifling itself is fine and the problem is likely that my old bore is still fouled. He brought up that 10,000 pellets with light cleaning is going to leave a fierce coating and I maybe haven't got the bulk of it yet. However he's coming at it from a PB perspective so it may not apply.

Here's a comparison between the two crowns. The picture isn't the best but it was the best I could do. Barrel on the left is my old barrel, right is the new barrel. See how the fresh barrel from Crosman is black on the inside? See how my old bore on the left is not as dark with strips of silver shining through? On a PB barrel it would be said that the dark color of my barrel is fouling and the silver is the bore's proper finish peaking through. Yet if Crosman's barrel is black then perhaps I'm rubbing some sort of coating off of it. Not the rifling itself but whatever coating may come on it from the factory.




What it really comes down to is whether the inside of my barrel should have that dark coating or not. I think that's where the rubber hits the road. If its supposed to be there, its possible that I'm basically enlarging the bore by removing it. Not ruining the rifling per say, but I'm making the bore too big for the pellets to contact it. For the barrel to then be any good I'd have to start using slightly off-sized cast bullets after I've evened out the inside by removing ALL of the black.

On the other hand, I do distinctly notice that pushing pellets through the old bore makes a lot of turbulence especially near the middle of the barrel. The new barrel is completely without resistance except for the first push into the bore and then right before the crown. Not sure what this means. I guess it leaves open the possibility that the old bore is still fouled.

I've got a full barrel of lead solvent soaking that I'll leave in for 24 hours. I won't post any more updates unless I have some news. I'm going to hold off installing the new barrel until I work with this old one some more. Although the new barrel was cheap I'm very curious to get to the bottom of this. I also feel like I have the barrel transfer port just right. I drilled out the replacement port tonight and it came out a touch smaller in spite of using the same bit. If at all possible I want to save the old barrel.

[Bullfrog]

I'm super stoked. I can report some definitive results after a week of testing, although there's still some mysteries to solve.

First, I'm extremely pleased with how my gun is shooting. I replaced the barrel (more on that later) and conformed the barrel port to .18. Accuracy has been restored (with a major caveat, again, I'll talk about that further down the post). Here's my final tune:

31 grain Barracudas
2700psi
943.4
942.8
949.1
950.0
941.4
945.5
945.7
945.5
954.1
939.6
946.0
952.1
948.4
953.5
944.9
Error
891.0
Ending pressure 2000psi

17 shots. Average 943.3fps, 61.3fpe. Average 41 psi used per shot. 1.24 efficiency.

My gun behaves almost as if it is regulated. I can't explain that, but the behavior is consistent. Dime holes at 20 yards. I'm a happy man. Having equal sized transfer ports definitely fixed my efficiency problem. I'm comfortable that I could use bstaley buffers and lower my power if I wanted to and maintain or gain efficency. 

Now here's what I've been doing this past week and some observations I've made. I spent the better part of the week cleaning the old barrel a couple of hours a day. Between scrubbing sessions I'd let is soak in whatever I could find. Hoppes, Bore Scrubber, Lead Remover, whatever. I kept getting soot out and mid week I noticed that the rifling returned when I pushed pellets through. As where there was hardly any rifling when I started, clear and defined rifling appeared all the way around the pellet. This encouraged me, but I couldn't get over wondering if I had now cleaned the barrel and was after a point simply removing bluing from inside the barrel along with fouling. The only thing that kept me going besides seeing an improvement in the rifling was the rough spot in the middle of the barrel. When I'd push a pellet through in the middle of the barrel I'd feel turbulence for a few inches. I knew that wasn't right. It also bothered me that the new barrel was definitely choked for the last couple of inches. I figured this would slow my pellets down. My old barrel doesn't' feel choked at the end. The rifling on the new barrel is definitely deeper but I attribute that to the choked crown. Between the crown and the breach there's virtually no resistance when a pellet is pushed through. Only at the crown does it take effort to push the pellet through. Compare that to my old barrel, where there is no resistance until the middle of the barrel, where it feels like turbulence and a slight resistance at the crown.

However, by Friday I realized I was slowly surpassing my costs in cleaners and brushes than a new barrel itself. Therefore I re-drilled the port in the new barrel to match the .18 port in my old barrel and installed it. The results were... the same. Pellets flying wildly. And yet sometimes I'd get a good group. Then I saw the pattern. Whether it be high velocity or low velocity, my pellets would scatter wildly when shot at 3200psi. My good groups with both the old and new barrel were always at the low bottom of my shot curve, even if velocities were still high at the bottom of the curve.

Therefore yesterday I tuned for a low fill, high velocity tune. Viola, complete accuracy restoration (with the new barrel mind you). Retuned for 3200 fill. Pellets sprayed again. It was clearly consistent. Tunes below 3000psi gave me accuracy, tunes above 3000psi were wildly inaccurate. I've almost always tuned my gun for 3200 fills and never had this problem until I drilled the ports out. Surely the bigger ports must have some effect on the pellet insofar as a fast, short puff of high psi air must be destabilizing the pellet. I can't explain it any other way and I can't understand the physics behind that. Yet the data shows a clear line in accuracy or lack thereof between high and low pressure fills. This also explains why I'd have the fluky good groups with my old barrel when all of this started. I noted then that the groups tightened up on the low side of my bell curve but I dismissed the idea when I turned the velocity down at 3200 psi and still got bad groups. I was still thinking in terms of velocity not pressure.

Wherefore, I can't answer what if anything was ever wrong with my old barrel. Maybe I cleaned the heck out of it for no reason. And yet surely it must have been fouled for the rifling to improve greatly over the course of the week. I'm going to continue to work on that old barrel, although I'm going to now do it at a leisurely pace. I'm happy enough with the performance of my new barrel.

Here's another observation. My velocity screw kept getting blown backwards during my shot strings. Because my velocity screw's end is concave, that means I got varying degrees of blockages during my shots. I would see this happen when my velocity would drop 100fps between shots. I finally had to blue loctite it in place. It seems to be holding. This told me that using the velocity port to restrict air isn't the way to go for my particular setup. I always lost power and air whenever the velocity screw was blocking the port.

Thanks for following along. The nature of my old barrel and how much cleaning is too much is still open to debate. I hope to have more answers as I work on that barrel for the weeks to come. I'd like to see if I can get that turbulence out of the center. Yet I'm not in a hurry to replace my barrel again. This new one is doing just fine. I don't want to tear into my gun again for anything the way its shooting now.

By the way, it is SO much quieter shooting the gun at the lower pressure. I have my hammer set about 6 turns CW and my #10 hammer spring about 3 turns CW. I'm using grey threading tape to keep the hammer from slipping. 

[Rdsail]

Very nice string! I think eventually I will start using the Cudas for hunting. I just have 9 tins of JSBs kings to go through. The accuracy of the kings is signicately betting than the cudas but I'm still getting 1in groups with the Cudas at 40 yards. Which is fine for hunting. They are also a lot more efficient. Right now I'm shooting JSB at 900 fps or about 45 fpe. The cudas at this same setting give for 50 fpe.

I'm feeling to 2700 and taking 16 shots. This give me a ES of about 12fps. My set up is producing very similar power as yours. Although, my porting is less than yours.

As far a accuracy an PSI, I notice the same thing with mine.

The main difference I have is that I'm using 12.5# spring. To get the power I'm getting is about 1.25 turns. I like that I don't have to have that much preload on the spring.

[rsterne]

I thought there was a setscrew to lock the position of the velocity screw, no?....

Bob

[Bullfrog]

I thought there was a setscrew to lock the position of the velocity screw, no?....

Bob

There is, but it doesn't hold. It blows back with the velocity screw. I had to loctite both the velocity screw itself and the set screw separately and then use the set screw to line up the concave velocity screw just right.

[Bullfrog]

Very nice string! I think eventually I will start using the Cudas for hunting. I just have 9 tins of JSBs kings to go through. The accuracy of the kings is signicately betting than the cudas but I'm still getting 1in groups with the Cudas at 40 yards. Which is fine for hunting. They are also a lot more efficient. Right now I'm shooting JSB at 900 fps or about 45 fpe. The cudas at this same setting give for 50 fpe.

I'm feeling to 2700 and taking 16 shots. This give me a ES of about 12fps. My set up is producing very similar power as yours. Although, my porting is less than yours.

As far a accuracy an PSI, I notice the same thing with mine.

The main difference I have is that I'm using 12.5# spring. To get the power I'm getting is about 1.25 turns. I like that I don't have to have that much preload on the spring.

I highly recommend the Barracudas for hunting. I've killed lots of coons with them. They kill coons better than anything else. I would have thought a polymag blowing up a coon's brain would kill them the cleanest. Yet there's something about that hard cuda tearing through them that puts them down better.

I'm glad to see I'm not the only one to observe a relationship between PSI and accuracy when dealing with custom ports. I'd like to know the physics behind why a higher PSI blast is destabilizing the pellets, even when the blast is small enough to send the pellet at a lower velocity. I don't understand why 750fps at 3200psi is less stable than 950fps at 2400psi.

[cambriajohn]

It maybe that the higher PSI causes the breech to twist on the air tube. It only has to move a thousandth of an inch or two at the breech to change your POI an inch or two at distance. Crosman now recommends fills less than 2600 PSI for accuracy. The breech is very thin at the bolt "Mouse hole" on the gen 1 marauder. The gen 2 has removed the mouse hole, I think to address this very problem. I've had good result with loosening the front breech screws 1/8 of a turn from full hand tight.

[HillGSA]

Maybe the quick high pressure blast is deforming the pellet?