Velocity Decrease

[jimhoodag]

The velocity has dropped on my 6.5 x .284 from about 2,950 fps to about 2,600 fps since May. The loading temperature was about 80 degrees versus 95 degrees when I shot them. I am shooting the same load, which is 52 g H4831 SC in Lapua brass pushing Berger 140 VLD's touching the lands. The rounds were loaded about a month ago and have remained in an air conditioned room and the rifle in the same room, but in a dry safe. I used my RCBS scales to test my RCBS electronic scales. The chronograph got wet, but it is correctly clocking my .243 105s at near 3,000 fps.

One possible cause is that I have changed from a vice type rest to a front rest and rear bag. I don't think the give in my shoulder from the recoil would account for a 300 fps decrease. Another variable is that the chrono was about 8 feet from muzzle instead of 10 feet in the past. Still my .243 was clocked correctly at the 8 foot position.

My Shilen match, select heavy sporter barrel has about 500 rounds down it. It has been kept clean, but there could be some throat erosion. My Lapua brass has been reloaded about 5 times and has never been annealed. The bushing on my full length resizing die creates about 1/1,000th of neck tension. The necks have not been turned yet. I am going to anneal and turn necks. I have used Sweets and KG keep copper out. Although I have thoroughly cleaned the carbon out of the barrel, I have not scrubbed the throat area as well as I probably should have. My guess is this or lack of annealing could be the problem.

Any suggestions would be appreciated.

Jim Hood

 

[bheadboy]

rest should not significantly change velocity, neither should carbon decrease velocity, but increase pressure and velocity. the move from 10 ft to 8 would also give a chance of the muzzle blast causing erratic crono trigger, I always place mine 15 ft out to eliminate that variable,


if the Crono is ok with other rounds then the ? becomes one of why no muzzle blast effect, easy the 243 has less than the 6.5x284.

I would think it is a function of the placement of the crono, try both calibers with it 15ft from muzzle, and I think you will be fine.

Bob

 

[jimhoodag]

Thank you for your well thought out responses. I bet you're correct. I have a muzzle break on the .243 so I can watch bullets impact. That should also reduce the muzzle blast. I am shooting perpendicular off of a levy, which is why I had to shorten my chronograph distance. Will test at 15 feet. Thank you.

Jim

 

[MagnumManiac]

Have you checked POI at your known zero?
This will be noticeable if velocity has dropped by 300fps.

Just a thought.

Cheers.
:

 

[jimhoodag]

The point of aim dropped maybe an inch at 300 yards. I haven't run calculations to see if 300 fps would equal an inch at 300 yards, but I will thanks. I have not had time to try moving the chronograph out further yet.

 

[savagedasher]

If you know your scope setting you use at the longest range you shoot .put it on that and see where the impact is. I use the impact difference for testing BC. not for actual numbers but the actual difference.
Same process work when checking bullet speed . If the bullet drops low with the crony telling me the bullet is 70FPS slower the crony is working correct. Larry

 

[dbramley]

Pull a bullet and check the powder. Not sure how you clean your cases but a friend had soap contaminates which held moisture and caused powder to form a large clump.