Reducing Velocity Variations?

[NYDAN]

I have a couple of loads that look very good for my 270 Win. However, the extreme velocity spreads are from 35 fps to 50 fps. I think I have read where it is considered desirable to have an extreme velocity spread of less than 1%. So, for 2900 fps muzzle velocities, it seems I should want to achieve velocity variations of less than 29 fps. My loads are using 150 gr. CTBST, 140 gr. AB and IMR 7828 which is probably one of the slower powders for the 270 Win.

I would like to able to shoot 400 yards with confidence. I definitely will NOT be shooting beyond 600 yards. Perhaps, velocity variations of 40 or 50 fps won't matter at the distances I will be shooting at deer?

Are there standard techniques for reducing velocity spread such as using magnum primers or utilizing a slight crimp? Or, is it a case of changing powders until you find both accuracy and velocity consistency?

 

[Oldtrader3]

I adjust loads and COAL until I get what I want, then I stop.

 

[Bruce Mc]

The large deviation in velocity could be a result of how you are shooting your groups. Are you consistent in letting the barrel cool down, are the ambient temps high and possible causing some velocity issues with your choice of powder, etc. It may not have anything to do with your hand load. I've been messing with some Reloder 22 in the .270 with a 130 Partition and have experienced some pretty good velocity variations but the temps have been in the 90s and its been tough to cool down. At the same time I shot some groups with H-4831sc and didn't get the large variations in velocity, same ambient conditions as the other.

 

[NYDAN]

The large deviation in velocity could be a result of how you are shooting your groups. Are you consistent in letting the barrel cool down, are the ambient temps high and possible causing some velocity issues with your choice of powder, etc.

Bruce, I try not to let the barrel get hot. I only shoot when the temperatures are less than 80 degrees, I stay in the shade. I space out the shots by checking each shot in the spotting scope, writing down the velocity, noting the location of the hole, and other delaying tactics. I walk out and change targets between each group. I also try to let the rifle and ammo stabilize to ambient temperatures before I start.

I have noticed higher average velocities on warmer days. But, I haven't noticed greater velocity variations on warmer days. Frequently, it seems that my first shot through a clean barrel is slower than the average.

 

[Ridgerunner665]

Frequently, it seems that my first shot through a clean barrel is slower than the average.

Leaving oil the barrel?

If so, it reduces friction, which reduces pressure....which reduces velocity.

Pull a tight dry patch through the barrel before doing any shooting.

 

[Rol_P]

Hi Dan, IMR 4350 was my go to powder for my .270 Win. Are you weight sorting bullets? I found .5 grain variation in a new box of Nosler 180 Accubonds that I opened this morning.. Don't know if this is significant or not. I also switched to CCI BR2 primers. I fired 3 groups @ 100yds last Sunday and they averaged about 1.2 - 1.3" which is an improvement.

I am playing with the seating depth now in search of a sweet spot.. Good luck, Rol

 

[OU812]

How are your 3 shot groups looking? Do they vary or pretty consistent? If they are acceptable to you I would not worry about it.

 

[HOGWILD338-378]

http://www.gunwerks.com/Long-Range-Univ ... e-Shooting

Nice article on your subject especially in the last paragraph and I am sure these guys understand ballistics better than I do which would not take a lot but hope it helps ya !

 

[jimbires]

go to http://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi and plug in all your info . see what the chart says 50FPS will cause at 400 yards , I'd say an inch or two . now at 1000 yards it will be significant . do some shooting at 400 yards to see how much vertical your groups have . this will be a way better indication of what's going on than depending on chrony readings . chronographs are not 100 % accurate , they are known to lie . a couple of things I've found are , I like my case powder fill to be in the 90 - 105 % . I think annealing my brass has helped my extreme spread . I do not recommend crimping your bullets , this is another variable .

 

[NYDAN]

go to http://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi and plug in all your info . see what the chart says 50FPS will cause at 400 yards , I'd say an inch or two .

Jim, you are correct. A velocity difference of 50 fps for the 270 win with a muzzle velocity of 2900 fps is an inch or less or 400 yards. Thank you for your insight and that link. I will save it to my "favorites" folder and make use of it from now on. Dan

 

[Partisan]

Dan;
You didn't mention if you are sorting your cases by capacity. My experience is that capacity variation is the biggest single contributor for any given load combo to velocity variation.
That being said, you can see from the external ballistics the vertical drop difference at 400 yards is less than an inch for a 50fps spread.
It is however easier to tune loads for accuracy (seating depth changes, etc...) if velocity difference is small. One less variable to deal with.

 

[NYDAN]

Dan;
You didn't mention if you are sorting your cases by capacity. .

Partisan, Thanks for suggesting I check that. Yes, I have been sorting brass by weight. I group by 1/2 grain increments.

I think I am going to go back and try the IMR 4831 again. The one group I shot with it only had an ES of 12 fps. I now have a better idea of the bullet jump the 140 gr. AB likes in my rifle. I am thinking I can fine tune with that powder now.

The grass (hay) in my fields is too tall now to do any shooting. It has been too wet to mow where I normally shoot. Plus, this is my busy time of the year with brush hogging the hillside and hill top fields that I can't lease out to farmers.

Dan

 

[RiverRider]

I think there's a lot more to seating depth than just tuning to vibrations. I think it has a lot of effect on ignition consistency and can be used to promote accuracy for that reason.

 

[Oldtrader3]

I always adjust my sighting in for 300 yards and put it where it should be at that distance before I go hunting. The chrono is not always the issue, sometimes it is the BC of the bullet being pushed a little.