Home Forums Hunting & Shooting Ask The Gunwriters Browning BAR 7 Mag velocities way under book/QL

I just assumed .009 difference both in length and diameter between the "match" chamber and the "sloppy mess" chamber, calculated the ratio of the fired case capacities (roughly - I assumed the case was a cylinder etc.) and then used QL to tell me what would happen to the velocity with that extra space.

Clever

+1 I like that. It appeals to my geeky engineer side. 🤓

After reading all the posts again, I come to the conclusion that the published data you used is likely “optimistic” using a test barrel of 26” , not the likely 24” BAR standard for that caliber. There’s 50-75 fps of your variation. I have had many loads that fell 100 fps or more below book velocities. Give yourself another 50-100 fps for chronograph variation and you are right there with your 200 fps deviation. I would bet most 160 or 162 gr factory loads would give you about 2,900 fps or so in that rifle. Try 66.5 gr of RL 25. That should put you in the 2900 ballpark and work up from there.

The quickest, easiest test I can think of to confirm a generous chamber as the cause of the low velocity, would be to chronograph a couple of factory loads. If they also produce similarly low velocities, then you have a loose chamber and perhaps an extra-long throat. If so, one can then increase the powder charge above the book max until reaching a more satisfactory velocity. If resulting velocity does not exceed book values, corrected for barrel length, and perhaps a bit of a fudge for the semi-automatic action, then pressures won't exceed book pressures either.

Ironically, loose chambers are produced when a brand-new chambering reamer is put into service. And tighter (faster) chambers are produced by the worn out reamer about to be discarded.

A buddy of mine has a bar in a 7wsm. Freaking federal ammo was as fast as it was supposed to be in that gun but like a lot of federal ammo it was unreloadable because of loose primer pockets. When we tried handloaded ammo we were way slower than the books said. Never did try to get to the bottom of it.