Why are groups impacting lower on target as muzzle velocity increases?

[Cassidy]

To tune my rifle barrel, I imprinted (using an ink stamp) 9 small (1/4 inch bulls eye, 1 inch ring and 2 inch ring) targets on one large 3-4 foot sheet of paper at 100 yds. I gradually increased the powder load by 0.3 gr for each target, targets numbered 1 to 9, four shots per target. I went through a series of rounds which included initial bore sighting, then barrel cleaning and 2 foulers before each series of 9 shots, serially one at each of the 9 targets (with serially increasing powder loads for targets 1-9), for 4 series. I reversed the target order for the 2nd and 4th series to account for barrel temperature changes during the 9 shots. I used a Chrony to obtain the muzzle velocity for each shot. I obtained the data I needed to select the least vertical group size, and the least wind effect (least horizontal group size). However, I noticed that as I increased the powder loads, the impact group was highest (about 3 inches high relative to the bulls eye), for target #1 (with the least powder load and the least muzzle velocity) and the group gradually dropped to about 2-3 inches below the bulls eye for target #9 (with the greatest powder load and the greatest muzzle velocity). What I don't understand is why the fastest muzzle velocity had the lowest group impact point; I expected the highest muzzle velocity to have the flattest trajectory and therefore the highest group impact point relative to the bulls eye, and just the opposite happened. Anyone know why?

 

[treeman]

if the bottom of your buttstock is not paralell to centerline of the bore the slower rounds are in the barrel longer giveing the rifle more time to recoil rearward allowing the barrel to tilt upwards. if you hold the rifle tighter into your shoulder it will lessen this effect

 

[Clod-NC]

Hi Cassidy,Hi Treeman

Thanks a lot for your post Cassidy,really. I've had the same question puzzling me ever since I saw that at a 100m,my poi for my handloads,which I knew were hotter than the factory rounds I was sort of comparing them with,were way higher than the factory ones,and I couldn't understand why,and still can't at the moment...So I'm keen on any wise man advice,or technician,or science oriented member here that will bring light in my head...I'm about certain it will be so simple,that I'll mentally kick my butt for being as dumb as to not being able to figure it out myself...So thanks for asking Cassidy,really.I love this forum....

 

[rocketron]

I'd say it has to do with the arch in the bullet path. at some point (distance) they will cross over each other.

 

[FroggyOne2]

I read on here a long time ago, that as you do your testing, (believe was a ladder test thread) as your loads are increased and your bullet impact rises and then one shoots low (provided that your tech is consistant) that this is a first indication that you have reached pressure.

I have seen this happen in the past when shooting ladder test and a lot of times the next or the second next shots would start having shiners on the bottom of the case. I usually don't take a set of calipers and mics with me to the range to measure the web.

 

[treeman]

this phenomnon (faster groups landing lower) will also go away if you shoot at 300yards

 

[Alex Wheeler]

Thats where the barrel was pointinging when the bullet left the muzzle. The barrel has harmonics and it whips around. If you shoot a long sting of groups increasing powder each group you will watch the poi go up then come back down. This is normal. And like was already same, this will go away at longer range because trajectory starts to take over. Also I bet your best groups will be at the highest or lowest poi when the barrel stops.

 

[Raythemanroe]

Do your ladder at 300yds and it should change

 

[BoydAllen]

Just out of curiosity, what are your rifle's caliber, highest velocity, and barrel length?

 

[jsthntn247]

As others have stated, your barrel whips before the bullet exits the muzzle. When the bullet leaves the muzzle during the whip cycle determines where it hits on paper

 

[allenn]

http://www.border-barrels.com/articles/rimfire_accuracy/tuning_a_barrel.htm

http://www.varmintal.com/a22lr.htm

 

[Cassidy]

Boyd - to answer your question, the rifle is a Rem 700 XCR compact (20 in barrel) shooting 308 175 gr berger vld in lapua cases, vv N550 powder. Muzzle velocity increased from a range of 2322-2375 fps for target #1 (n=4 shots) to 2508-2595 fps for target #9 (n=5 shots). The groups clearly gradually dropped from about 2.5in above bullseye on target #1, to 2in below bullseye for target #9.

I have been tuning another rifle, 28.5in barrel, 6.5x47 lapua, and have tried 130 gr berger vld, H4350 powder, in the first tune test (which I was not happy with), and scenar 123gr, vv N540 powder in the 2nd barrel tune test (results for the 2nd tune were equivocal due to slightly moving (1/2 in) target in the wind; will have to retest). In the second test (more extensive test), muzzle velocity increased from 2967-3027 fps in target #1 to 3065-3240 fps in target #10 (optimal load was 3041-3080 fps). Both 100yd tests. However, interestingly, in both of these barrel tuning tests the poi did not change relative to the bullseye - it stayed right on target for all 10 targets regardless of powder load; the poi did not drop with increasing muzzle velocity like the rem 700 did. These data suggest to me that this is not a barrel harmonics matter.

Not sure why this is happening with the 20in barrel and not the 28.5 in barrel. Your thoughts are welcome!

Cassidy

 

[BoydAllen]

Given the stiffness of the 20" barrel, and the velocities involved, I would have to guess that it may be a buttstock angle on rear bag thing. How much shoulder are you putting into the rifle? It would be interesting to compare a couple of three shot groups with the two loads that gave the greatest difference in elevation, that were shot with a front rest only. Admittedly they would be larger, but it would by their vertical positions that I would be most interested in. Is the .308 coming off of the front bag during firing?

 

[Cassidy]

Boyd - the .308 Rem 700 20in barrel has a harrison bipod which I hold down with my left hand, and I have the butt firmly/comfortably in my shoulder. The BR 28.5in barrel rifle is on a BR front stand and supported with a BR bag under the stock butt so things slide like a BR rifle is supposed to when fired. I touch only the trigger when firing this BR rifle - my shoulder is about 1/2in behind the butt when fired. Will have to test fire some rounds as you suggest to figure this out, but wont be able to do it right away.

Cassidy

 

[BoydAllen]

...with the additional information, even though you are holding it down, you may be seeing some bipod bounce, that may not take the feet off of the top, but just be a mater of the degree of unloading.