Re-tuning an already tuned AT44

[Loki_762]

I recently traded my Crosman 1720T for an AT44, which the previous owner had "tuned" to shoot 14.5 ft/lbs.  This what exactly what I was looking for, so I was hoping I wouldn't have to mess with it.  So, I got it home, put it over the chrony, and instead of putting out 14.5 ft/lbs, it was giving me 11.5 ft/lbs, and I was only getting about 20 shots  before the velocity began to drop pretty severely.  After those 20 shots, the gauge on the gun told me that 20 shots had cost me 50 bar of air, which seemed mighty high for 20 shots at around 765 fps.  So, I decided to tear it down to see what the tuner had done to it, to see if I could undo some of the  "tune".  Now, this was my first time inside an AT44, so maybe I missed something, but it looked to me like the "tune" consisted of putting in a more constricted transfer port.  Period.  Nothing polished, no aftermarket stuff that I could see, beyond the transfer port.  So, I opened it up to 3/32nds, to see what that would get me.  Got me to 925 fps with JSB 8.44 grainers, and around 885 with 10.34 grainers, which was right around what I was looking for.  Still had a crummy shot count, though, getting around 20 shots before the velocity took a steep and continuous downward curve. 

The next thing I did was to plug 5 of the 6 holes in the valve, and then fill in the remaining space with JB weld.  I shimmed the valve face with a very thin piece of plastic so that 6th hole lines up perfectly with the transfer port.  Gassed it up, and put it over the chrony today, and the first shot was 865 fps at 200 bar with a Crosman Destroyer.  Slower than the last time, but I figure that is because there is less air flow, which is easy to fix by cranking up the hammer or opening the transfer port more.  It held pretty steady for the first 10 to 15 shots, and then it began to get erratic - the velocity would jump 5 fps, drop 15, jump 20, then hold consistently for a few shots, then jump around some more.  Then at around 30 shots, it began the drastic drop in velocity again.  It did this over the course of two fills up to 200 bar, almost exactly the same each time.  I had read that doing valve plug mod would get me almost 50 shots, so I am a little stumped.

So, here is what I am thinking - it is a little starved for air, so I need to get that transfer port open a bit more to let the air flow.  I will probably try it out at 7/64ths, and if that doesn't work, 1/8th.  I know that many people recommend to open the transfer port, valve port, and barrel port all up to 9/64ths, but I don't want too much power.  I am trying to keep this thing at a maximum of 925 fps with 8 grain pellets, so I don't want to open everything up all the way if I don't have to.  I also wonder if perhaps I should put in a different valve spring - seems like it is wasting air if most people who have done the mods I did are getting 20 more shots.  Perhaps part of the "tune" that was done for the previous owner involved a softer valve spring, and too much air is rushing out.  I have no idea what to do about the erratic velocity, though.  Perhaps figuring out the valve spring and port size will fix that.  Other than a regulator, (which will likely eventually happen anyhow,) does anyone have any other advice about what they think I should try? 
Thanks in advance,
Chris

[Bwalton]

925fps is a little hot for that pellet 890 is a good speed. Where did you see 50shots at 925 at? Is this the long that you have or the short? Stick with the  heavier pellet and go for 890fps is what I would do.

[SpiralGroove]

Hey Chris,
If you want, come over my house and we can look at it.
Kirk

[pllagunos]

I would definitely take the chance and go with Kirk to check what you can do with the AT44, also check if you have the Hammer debounce spring in place because when I first disassembled my at44 I forgot to put it back and the strings had poor shot count.

[Loki_762]

Thanks for the replies.  Baxter, it's a short, and after going back and looking, that 50 shots I read about WAS with a regulator, not without.  Sorry about that.  Still, 30 to 40 shots with a decent curve at around 900 fps should be doable, right?  I took it apart again today to make it externally adjustable (stock mods), and opened up the transfer port to 7/64ths.  Not a huge increase, so we'll see what we get.  I haven't had a chance to run it over the chrony yet - I'll likely get to that tomorrow.  If I can't get a nice little velocity curve over at least 30 shots, I will give you a holler, Kirk, and hopefully we can set up a time to meet up. 
Thanks,
Chris

[Hobbyman2007]

I would try a full string from 200bar to 100bar to see where the sweet spot is first . Then find a happy place to fill to , once that is figured out fine tune with the preload on the hammer spring . Your porting should be pretty close at near 75% of bore size. I managed to get 29 shots at 50+ fpe with a .25cal .

[Voltar1]

Thanks for the replies.  Baxter, it's a short, and after going back and looking, that 50 shots I read about WAS with a regulator, not without.  Sorry about that.  Still, 30 to 40 shots with a decent curve at around 900 fps should be doable, right?  I took it apart again today to make it externally adjustable (stock mods), and opened up the transfer port to 7/64ths.  Not a huge increase, so we'll see what we get.  I haven't had a chance to run it over the chrony yet - I'll likely get to that tomorrow.  If I can't get a nice little velocity curve over at least 30 shots, I will give you a holler, Kirk, and hopefully we can set up a time to meet up. 
Thanks,
Chris

How did you determine that 30 to 40 shots with a decent curve at around 900 fps is doable?

[Loki_762]

Thanks for the replies.  Baxter, it's a short, and after going back and looking, that 50 shots I read about WAS with a regulator, not without.  Sorry about that.  Still, 30 to 40 shots with a decent curve at around 900 fps should be doable, right?  I took it apart again today to make it externally adjustable (stock mods), and opened up the transfer port to 7/64ths.  Not a huge increase, so we'll see what we get.  I haven't had a chance to run it over the chrony yet - I'll likely get to that tomorrow.  If I can't get a nice little velocity curve over at least 30 shots, I will give you a holler, Kirk, and hopefully we can set up a time to meet up. 
Thanks,
Chris

How did you determine that 30 to 40 shots with a decent curve at around 900 fps is doable?

I based this on the specifications on the Pyramyd Air site for a full power, untuned gun.  I figured that if I am making it more air efficient, and that if I am bringing the velocity down, I should be able to get at least the advertised shot count.  I have also read posts stating that people are getting that with .22 and .25 versions, so .177 seems possible.  That was the entire point of me attempting to mod the gun. 

[SpiralGroove]

Chris,
I can get 70 shots with my BAM B50 which has a 196cc air cylinder vs. the 185cc for the Hatsan; further the BAM also loses about 30cc due to a regulator and uses CPUM.  So with efficiency at 1.0 or better, (IMHO) you should easily get 50/70 shots at 900 fps using 7.9 CPM.

I wouldn't accept anything less

[Loki_762]

Okay, I got pretty close, and I may just call it good until I can get a reg in there.  I opened the transfer port to 7/64ths, and with the adjustment backed all the way out, I am shooting the 10.34 JSBs at around 880 fps, the Hunter Extremes at about 895 fps, and the JSB 8.44 grainers at about 920 fps.  I still get some spikes, but I get them only with lighter pellets so far, especially the Destroyers.  Maybe the pop of air is just too much for them.  Regardless, I am getting a nice curve with around 20 fps variation for 25 to 30 shots.  Probably a few more even before POI shifts any due to velocity drop.  I'll call that a win.  Next I need to slap a scope on it and see what pellets it likes.  Thanks for all of the help, guys!
Chris