How is this possible?

[chas3stix]

I was visiting the Accurate Shooter site and came across info on the 30BR round.
The parent case is either 6mm or7mmBR. The case is necked up for 30 caliber bullets.
Here's the part I don't understand.....they're claiming 2900 to 3000 fps with the 30BR using33 Gr. of H- 4198 powder. For a comparable load with the 7mmBR, the maximum velocity is 2685 using 28.5Gr. of H-4198. (both rounds shooting a 115Gr.bullet). So how do they get the extra fps?
I'm no ballistician or math whiz....would appreciate someone out there to explain this to me.Thanks in advance.
Chas

 

[backwoods83]

Well, being that the 7br is .284" diameter bullet and the 30br is .308 and they are both likely flat base bullets so 2 simple reasons are 30 cal has a larger base to push against and a much shorter bearing surface, which leads to less pressure in the 30 cal with equal powder charges, so you can use more powder and get more speed.

 

[liltank]

Faster powder will work better due to the larger throat diameter too.

Tank

 

[Mikecr]

+1 Backwoods
And 4.5gr more powder..

 

[chas3stix]

Thanks for the explanation.

 

[RT2506]

I think they are using a pretty fast twist barrel also.

 

[liltank]

They state a 1:14 or 1:15 twist. That will work up to about a 125grn bullet.

Tank

 

[Mikecr]

Yeah they use very slow twists(out to 18tw), which is a great contributor to their performance.

 

[KRP]

I think they are using a pretty fast twist barrel also.

"Tactical" guys might be, shooting heavy bullets at subsonic velocities through suppressors, but the Accurate Shooter guys are probably shooting Benchrest for score with slow twist barrels and light bullets.

 

[Trickymissfit]

They state a 1:14 or 1:15 twist. That will work up to about a 125grn bullet.

Tank

actually the big rounds with a 30BR are the 115 thru 120grain in a 16 or 17 twist barrel. Pay a visit to the BIB Bullet website. The 17 twist is a little temperature sensitive, and probably the 16 would be better in cool weather
gary

 

[vendetta333]

with ballistics like that, y does this not sound like a great hunting round to anyone? sounds to me like a little under 243 to 260 performance but with waay less powder! sorry if i sound like a tard. plz correct me. be gentle lol lightbulb

 

[liltank]

with ballistics like that, y does this not sound like a great hunting round to anyone? sounds to me like a little under 243 to 260 performance but with waay less powder! sorry if i sound like a tard. plz correct me. be gentle lol lightbulb

I really considered it as such. The problem is getting decent hunting bullets that are non varmint in those weights.

Tank

 

[backwoods83]

As mentioned these 115 and 120grn pills like the BIBS are flat base, no boattail, the bc is not that good and they are used for 1, 2, and 300yrd benchrest where a high bc bullet is not a necessity, that's why they are not good hunting rounds, now a 8 twist 6br with 105 bergers is a good whitetail/antelope stopper.

 

[vendetta333]

well, i was thinking as a great <300yrd deer rifle, but i did not think about bullet construction, silly me, you're right they are mostly varmint bullets. glad i wasn't totally off though.