I have an inquiry as to the .308B chamber dimension used in BHW .308 chambered barrels.
My barrel is a .935" AR.308 1-11 twist. And I noticed all my brass is leaving the chamber with a goofy radiused hump at the shoulder/neck junction.
Let me first say that it isnt causing me any issues with the actual relaible function of my rifle. My issues are, #1) how will this affect the life of my brass? I am bump sizing and this small bulge is being pushed back in every time (I have a picture that features a neck sized case, but cant figure out how to upload it).. And #2) is this part of the reason im having a hell of a time getting this barrel to group under 1.25 MOA?
Aside from that its purely superficial to me, though I would prefer it werent there.
I bought a 308B barrel in June of 2014. I assembled my gun and it has operated flawlessly so far, but my grouping capabilities are horrible, despite using a Caldwell lead sled. I'm using a JP enhance 308 bolt. I recently checked my seating depth and find that for Hornady SST (2.890 COL), Barnes ttsx (2.965 COL), and berger classic hunter (2.910 COL) the seating depth far exceeds any usable loading length. I also accidentally loaded some brass that wasn't trimmed sufficiently (I was using a trimmer where the stop fit too deep into the primer pocket secondary to a large chamfer on the primer pocket). These brass were well beyond the 2.015 maximum length and chambered in my gun, although a very tight fit, and I find that if I trim my brass to the maximum length my groups get better. I am concerned that my barrel has been chambered to deep to allow for best accuracy. I've tried everything I can think of to overcome this including using Federal Gold Match ammo, which frankly my reloads shoot better. I have the barrel mounted on a DPMS upper w/ DPMS barrel nut and free floating fore guard. I am using a BRO drop in trigger w/ 2.5 lbs pull and look through a Nikon M308. Do you have any thoughts on this? I put quality parts on this gun and carefully assembled it. I should be getting much better grouping.
I've seen distance to lands like that in most chambers in 308 done to SAMMI spec. Those bullets are pretty pointy. Loading them as long s you can often does the trick. The Barnes would want some jump from what I hear as I don't shoot them. How terrible are the groups?
As to case neck length all chambers are cut routinely at least 0.010 longer than the speced length as an industry standard. I had one factory 308 Remington that had the shelf about 0.030 longer. It ate anything I could feed it.
Did you square the receiver? Also while I have used sleds quite a bit in the past with good results I have found that a front rest and rear bag are mechanically easier to use. You might try a few with just bags.
I know that the AR10's historically are not the most accurate and have avoided them I am in the process if building one as I type this. I have a barreled upper in 308 I'll wring out as soon as I rustle up the rest.
The best firearm for self defense? Any loaded one will do.
My group sizes vary wildly. I may have two consecutive shots almost lay on top of each other, but the third shot will be 2-4, 6 in any direction. Or, they may all be 3 in off center in every direction, so that my effective group size is 6in or more. I have loaded all of these bullets to the maximum length that will fit in my magpul magazine (which I highly recommend) which is 2.825 in.
I did not square the receiver. I read another thread on this forum where a guy was having problems w/ grouping and he found that his barrel extension was unsupported in his receiver about half way around. He solved his dilemma by placing a shim between the barrel extension and receiver. I thoroughly inspected mine for a gap or anything that appeared off. I find that the barrel extension fits so well in the receiver that if I didn't know better I would think was machined as part of it. I will look for information on squaring the receiver to see if there is anything I can improve. I also intend to load some rounds that are closer to the lands to see if that helps. I will have to feed them one at a time by hand.
I used the sled because it take all(or at least most) my shooting error out of the shot. I tried using shooting bags in the past, but I find the sled holds the steadiest.
I've shot military surplus federal 147gr M80, federal power shock 150 gr soft point (used these to break the barrel in), federal premium gold medal 168gr SMK. All with the same results.
I've shot reloaded 125gr nosler balistic tips, 165gr Hornady SST (which has grouped the best so far), 168 gr Hornady AMax, 168gr Barnes TTSX, 175gr Barnes LR TTSX, and last 168 gr Berger Classic Hunter (which from my understanding is supposed to be less of VLD design). I matched the brass head stamps (in addition to number of times they have been fired), trimmed my brass by hand to matching lengths (I prefer 2.105 in as I find this length of brass gives my gun the best groups), weighed each charge down to a 1/10 of a gr on an electronic scale recalibrated frequently to exact matching weight, and seated each bullet to same seating depth (measured by COL). As I've stated earlier I tend to get better grouping w/ my reloads than factory ammo.
I'm going to purchase a receiver lapping tool from PTG and try squaring the receiver. I also loaded some Barnes 168 gr TTSX seated out to 2.915 in w/ 42 gr of XBR8208 (I have gotten my best groups with this powder and the Barnes bullets are just longer that tradition lead core bullets and will leave more bullet in the neck of the case to be supported) that will have 0.050in before the lands as recommended by Barnes. I'll let you know how they turn out.
My crown seems to be fine. I have a double star(?) compensator attached w/ a crush washer that I installed before I ever fired the gun. I have thought about removing the compensator to see if it makes a difference, but I really don't think its causing any accuracy changes.
That's quite a selection but I want to know the size of the groups. It is relative to say better or bigger. How big are the Federal GMM groups? How big are your best ones? How big are your worst center to center?
And I would pull that brake off to start with while I'm sorting this out.
The best firearm for self defense? Any loaded one will do.