Higher MV = Lower Point of Impact???

[ScottyB]

I noticed something unusual, or at least different than I had expected yesterday while doing some load testing. Using 9 sets of 3 rounds shot at 100 yards with charge weights varying from 40.0 to 43.4 (40.0, 40.5, 41.0, 41.5, 42.0, 42.5, 43.0. 43.2, 43.4) my point of impact actually got lower as the charge weight went up.

I would have thought that a higher MV would mean higher POI, but one of the other guys at the range said this was due to the higher charge weight rounds shooting flatter.

Bullets were 200gr Berger Hybrids
Powder was Varget
Barrel is a brand new 30" 10 twist Broughton

Can anyone provide any insight?

Thanks,

Scott

 

[itchyTF]

My guess would be barrel harmonics. How much vertical are you seeing between the .5 gr increments? If you have the space, you should try this at a longer range, like 300 yds.

 

[Laurie]

It's most unlikely that it's down to a flatter trajectory at only 100yd, although that affects POI at longer ranges. It's normally a result of the combination of bullet 'barrel-time' and the way the action and barrel flex as the bullet travels down it. Most actions see the muzzle move vertically as the bullet travels along it. If the muzzle is moving upwards during this process and the MV is higher, the bullet takes less time to travel from chamber to muzzle and so exits at a marginally lower muzzle position than with a lower load, lower velocities and longer 'barrel-time'.

I've usually found front-locking sporting ('hunting') rifles with skinny whippy barrels produce the opposite effect giving a markedly higher impact point with each grain increase in powder charge, while heavy barrel target rifles on stiff actions can often, but not always, see the bullet impact drop as MVs rise.

It's also not unusual to see the POI move laterally as pressures / velocities increase.

Incidentally, this 'barrel-time' related effect which is often called 'compensation' produced very marked and beneficial long-range results in British Lee-Enfield rifles. The rear-locking and not very stiff Lee action produced very marked 'positive compensation', that is the muzzle was rising as the bullet went through. As a result, a cartridge that produced a high MV exited earlier at a lower muzzle position and hit low, a slow bullet hit higher. At 600yd and beyond the slow bullet which would normally hit low on the target hit at the same elevation as a fast bullet thanks to the trajectory and muzzle position effects cancelling - or compensating for - each other.

 

[M14AMU]

It's awfully difficult to convince some folks that when a rifle fires....the barrel whips and vibrates just like a fishing rod....but they do! Bullets exiting the muzzle on different 'nodes' is proof positive!

 

[bozo699]

It's most unlikely that it's down to a flatter trajectory at only 100yd, although that affects POI at longer ranges. It's normally a result of the combination of bullet 'barrel-time' and the way the action and barrel flex as the bullet travels down it. Most actions see the muzzle move vertically as the bullet travels along it. If the muzzle is moving upwards during this process and the MV is higher, the bullet takes less time to travel from chamber to muzzle and so exits at a marginally lower muzzle position than with a lower load, lower velocities and longer 'barrel-time'.

I've usually found front-locking sporting ('hunting') rifles with skinny whippy barrels produce the opposite effect giving a markedly higher impact point with each grain increase in powder charge, while heavy barrel target rifles on stiff actions can often, but not always, see the bullet impact drop as MVs rise.

It's also not unusual to see the POI move laterally as pressures / velocities increase.

Incidentally, this 'barrel-time' related effect which is often called 'compensation' produced very marked and beneficial long-range results in British Lee-Enfield rifles. The rear-locking and not very stiff Lee action produced very marked 'positive compensation', that is the muzzle was rising as the bullet went through. As a result, a cartridge that produced a high MV exited earlier at a lower muzzle position and hit low, a slow bullet hit higher. At 600yd and beyond the slow bullet which would normally hit low on the target hit at the same elevation as a fast bullet thanks to the trajectory and muzzle position effects cancelling - or compensating for - each other.

Never heard it put quite like that, but well put Laurie, I can shut the computer off as I have learned something today, now I know why some rise and some drop,..Thank's
Wayne.

 

[XTR]

At 200 yards I've seen OCW groups that started above the POA, moved 3/4 inch down, then back up to the line.

I've also seen them move right till they were three inches from the POA.

 

[lloydx2]

Read Harold Vaughn's book. Rifle accuracy facts. Its a must read for any accuracy enthusiast

 

[Keith Glasscock]

If you are like me, you'll get the same confusing results at 300. I have been unable to get a readable ladder with that bullet ever! Now for the funny part:

I had a solid shooting load at 42.9 for the first ~250 rounds through the barrel. Suddenly, vertical and more vertical. I had to reduce the charge ~1 grain to get it back in line. Be forewarned, I have been unble to get the 200's to shoot at "max" in a 30" barrel like they did in a 26". With either barrel length, I get the same velocity. Maybe these bullets are the end of having to run long tubes for long range.

 

[RDavies]

Busdriver. I,m suprised your gun didnt get the velocity increase until 250 rounds. Mine (6,6.5,7mm) always got it between 100-150 rounds, though I have heard that some other 308s didnt get their velocity kick until more rounds down the tube.
As for low velocity rounds hitting higher. Yep, my 284s when tuned for 1000-1200 yds gave slight verticle at short ranges with the faster bullets hitting lower. At 1000-1500 yds, it got great verrticle. The strange thing is the long range load is .2gns higher than the mid range (500-800 yd) load, but thats the way regulation and harmonics work.