I'm right about to put in an order for a Pac-nor barrel, 20" chambered in 7mm BR. I'll be slinging medium to heavy grain bullets (140-160gr) so the twist will most likely be 1 in 9. My question is, how many grooves should I go with. The only other custom I had built was a 6.5-284, and got a 3 groove (not really my decision). Given my twist requirements, I could go with pretty much anything, including a 3, 5, or 6 groove barrel. I'm leaning toward the 3 groove (don't know why), but any input is welcome.
Accuracy wise you will probably never see a difference between all three.
I use 3 groove barrels in all my sub 30 cal high capacity rifles especially the likes of my Allen Mags simply because they offer increased throat life compared to the other designs.
This is because the land is wider and has more steel in it so it takes measurably longer to erode compared to a 5 or 6 groove.
Another benefit 3 groove is slightly higher velocity but other variations can cause this to vary from barrel to barrel so do not expect to gain 100 fps simply by using a 3 groove barrel.
From my testing and experience, I would say that 3 groove barrels copper foul less then the other designs simply because there are fewer edges and less distortion in the bullet as it passes through the bore.
Again, with top grade match barrels that are properly broken in, this will not be an issue with any of the designs.
Now for your little 7mm BR, which I think would be a kick in the rear in a rifle mind you, I would go with the 3 groove for the reasons above but to be honest, you will probably never burn the throat out nor will you produce enough velocity to lay down alot of copper fouling.
Still, I would go with a 3 groove just because [img]/ubbthreads/images/graemlins/grin.gif[/img]!!
Allen Precision Shooting
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Years ago I had a 7br rifle made by Rem out of their custom shop on a model 7 action. It was a great paper shooting rifle but lousy on the varmits just couldn't get enought velocity. With todays bullets and twist maybe different. Most of the pistol in 7br had a barrel about 14". You may want to look at the Dasher case for a 7mm should get pretty good velocity over the 7br and think it would work reall good with the 150 or 168 gr match bullets. I still shot the 22 and 6br. Well good luck.