Tuning Fiddling with the valve

[Khornet]

I watched Bob's video in which he says to tune to a one-hole 30 yd group, then go for 100, and if not good at 100, adjust the valve, and if no better, the hammer or regulator etc. What struck me most about it was the idea of using the valve adjuster to find the best barrel harmonic in an Impact. I'd been doing pretty well at what passes for long range in my backyard, 70 yards with H&N 23 gr .218 slugs, with 0.5" 5 shot groups the expected, 0.4" very common, and once 0.35", in the original stock pellet liner 700 mm. But of course I wanted better, so I bought a Superior (not heavy) liner, polished and installed it and set to work. Got it to just about equal my prior groups after about 200 tuning shots, then groups opened up, so I cleaned the bore that night. This AM, shooting over a chrono while re-seasoning, I found my velocities had risen a bit, from the previous 896 average to 910, but with ES only 7. Knowing from experience that a tight ES by no means guarantees accuracy, I fired five at 70 yd and got this, in still air:

Sheesh. So I closed the valve a 16th. Next five:

OK, progress. Closed it another 16th:

The 905 fps is just a guess, since I haven't chronographed it yet. Heretofore I'd only adjusted the valve to fine-tune velocity and ES. Guess I should stop caring about ES.

 

[biohazardman]

I used Bobs video to get the Impact tuned up as well. Makes things rather easy and once you have done it a couple of times you can do it in a lot less shots. Nice job on the tune by the way accuracy is there.

 

[Khornet]

Puzzled by the velocity gain going from STX pellet liner to Superior. I think the former is 1:19, while the latter is 1:26, so maybe that's it.

 

[tor47]

Wow, that is a nice group Just one question: I have seen it is advisable to make 3-5 dryfire shots everytime you adjust the impact, to let the springs in the hammer and valve settle, while fine tuning. Do you do that, or do you just shoot slugs right away?

I know when adjusting the valve adjuster one are actually twitching the spring some, as it is already under tension, so to me it make sence to take a few dryfire shots to let the twitch dissapear, before loading the mag.

 

[Khornet]

Tor, I always fire a few after any adjustment before going for group. I don't dry fire though, thinking that I want to put the system through the same forces as firing a slug. For hammer changes it's usually three or four, and for valve adjustment one or two.

All groups were fired with a single loader. That's all I've used for quite awhile for tuning. In fact, I haven't used a magazine in months. If this tune holds up I will test with mags. This AM at 15 degrees but no wind I fired two rounds to see whether zero had changed. Holes touching, zero unchanged.

 

[Khornet]

I did chrono this tune before checking zero this morning. Five rounds, average velocity 905, ES 7. Regulator 95 bar.

 

[tor47]

Tor, I always fire a few after any adjustment before going for group. I don't dry fire though, thinking that I want to put the system through the same forces as firing a slug. For hammer changes it's usually three or four, and for valve adjustment one or two.

All groups were fired with a single loader. That's all I've used for quite awhile for tuning. In fact, I haven't used a magazine in months. If this tune holds up I will test with mags. This AM at 15 degrees but no wind I fired two rounds to see whether zero had changed. Holes touching, zero unchanged.

Ok, thx. Seems like a good idea using single loader also, so every shot get exactly the same feeding, and to rule out other factors.

 

[Khornet]

At 0630 today, 9 degrees, zero is unchanged. However, at 100 yds this tune and slug only manages 1" to 1.25", or 4-5 times the size of the 70 yard groups. There was no wind, and I was as careful as possible about keeping level, but clearly 100 yards is a whole other level.