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

[Bullfrog]

Maybe the quick high pressure blast is deforming the pellet?

Couldn't say. Family Man is reporting the same inaccuracy problems at a higher fill pressure on his high-flow gun. Looks like a verifiable pattern. I'm wondering if it is barrel harmonics with the shroud. I forgot to report that last week in my accuracy testing I had one shot on a 3200 fill where the shroud got blown 2 inches down the barrel before the set screw caught again. I didn't have the set screw resting in the notch on the barrel, I had it just above the notch but tight (but I guess not tight enough). I wonder if that really strong blast is shaking the shroud and therefore the barrel?

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.

What I'm thinking about in terms of barrel harmonics might be the very thing you're describing. Its very interesting that Pyramyd Air's description for the Mrod now includes language stressing a 2600 fill or less for best accuracy. Hmmmm....

[cambriajohn]

I added a 0.625 od carbon fiber tube over the full length of my barrel. Then I put 10 or 15 o-rings evenly spaced down the length in order to use the shroud as a barrel stiffener. I sprayed silicone down the shroud but none on the carbon fiber or o-rings, this way they can be tight but have more friction to the barrel than the shroud when assembling. The shroud is already attached to the barrel by an oring at each end so this does not affect the free floating design. I do not attach the shroud to the barrel band as some have suggested as this IMO directly connects the barrel to the shifting air tube. I shoot single shot as I found the magazine reduced my accuracy. I made a single shot tray to make it easier to load. I shoot benchrest at targets so this mayn ot be the preferred hunting set up. When I purchased it my 25 cal Marauder would not group at 75 yards, now I get 23 out of 25 within 1 inch ctc at 75 yards. I use JSB 25.4 Kings at about 910 FPS.

[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.

I'm updating this thread to report some changes to my gun's performance.

First, my gun kept performance as in the above listed string for several weeks. However, I had to take my gun apart for something a few weeks ago (can't remember why) and when I got it back together I was unable to duplicate this amazing tune and my efficiency dropped significantly. I also found that after reassembly, my gun would waste air again with light pellets, a problem that had previously been eliminated. Finally, the regulator-like numbers disappeared and my gun began shooting on a normal bell curve again.

Recently Bob and Tim suggested to me that the reason that my gun might not be able to shoot accurately above a 3000psi pressure is because perhaps my .18 ports are too big and causing a little bit of the pellet's skirt to be exposed in the transfer port. When the air is released during the shot, the high psi levels might warp the skirt while low psi levels may not. That made a lot of sense to me. As I've already lost my super tune anyhow for reasons unknown, I figured it would be good for me to test this to see if this was the problem.

Well, I never actually go around to testing the theory directly. The way I would have done this would be inserting a pellet and looking to see where the skirt was at after removing the barrel. Instead of doing this, I ended up tinkering with barrel alignment and discovered some interesting things.

First, I have completely taken for granted that the barrel's intake port will line up exactly with the transfer ports every time you set the barrel in the breech. That's completely wrong. There is a lot of room for play when you fix the barrel into the breech. The screws in the top of the breech that hold the barrel in place are not so precise that they automatically align the barrel when you tighten them down. Its very easy to position the barrel so that the barrel's intake port can be off center in any direction while still allowing the gun to function. However when its off center it affects the guns' performance.

For example, I've found earlier this week that having the barrel a little too far back would cause my gun to shoot 8 shots at a max 70fpe with an efficiency of .9-1.0 from a 3000psi fill. However, getting the ports aligned as perfectly as I can eyeball it raises the efficiency on the same tune and power level to 1.7-2.0 from a 3000psi fill.

Here's the other big change. I can now fill my gun as high as I want and maintain accuracy. I've tested as high as 3200psi and accuracy remains.

I'm wondering now if I had my gun's barrel set a little too forward when I had it set on my super tune. Having the barrel set too far forward might have been causing the pellets to not seat as far up the barrel as they should and therefore their skirts to have been exposed during the blast. When I dissembled and resembled the gun it may be that I aligned the barrel differently causing the drop in efficiency. I can only test this by moving my barrel forward and seeing where the pellet is. However I've got my gun shooting decent again and I don't want to move it.

Currently I have the gun set for 8 shots from 3000psi to 2600psi. Instead of waiting between shots like I normally do, I shot all my strings today in rapid succession. With 43.2 Eunjins I'm getting an average in the 860s within a 4% spread for an efficiency of 1.2. With Barracudas I'm getting an average in the 980s with a 2% spread for an efficiency of  1.11. Those are good numbers and I can't complain, but I do miss that awesome low pressure tune with the Barracudas. I also don't like the wasted air with the lighter pellets. I though I had solved those problems. For the record, I've checked my velocity screw and every other variable I can think of. I just can't figure what I changed when I disassembled the gun other than maybe the barrel alignment. I can't rule out that perhaps I accidentally changed my hammer spring. I have several 10# springs and some of them are more worn out than others.

If anything it just goes to show how fickle PCPs can be, especially when you start tearing into them and modding. The only thing I can say with some certainty is that there was apparently was some zonky way I had my gun assembled or tuned that was causing crazy efficiency and power at low psi and whatever that was was probably also causing my accuracy problems at a normal power-tune fill pressure.

I will also note that I was able to get some tunes this week at 2700 fills with light pellets for 50fpe for decent efficiencies. However, on my current 70fpe tune with the Eunjins I'd only have .8 or .9 efficiency with light pellets. 

[rsterne]

When you tune for high power with heavy pellets, it is completely 100% normal and expected for the efficiency to go out the window with light pellets.... Tuning for heavy pellets requires a large increase in dwell.... Using that dwell with a light pellet keeps the valve open much too long, and much of the air is coming too late in the shot cycle (the pellet is too far down the barrel) for it to have good effect....

Bob

[Bullfrog]

I just reread my post. My tune efficency is 1.17-1.2, not 1.7-2.0.