I did a search, but didn’t find out much. This has me a little perplexed…
I just got my tax stamp and threw together my SBR to zero and function check. It’s all BCM (standard 11.5" barrel) with a Troy VTAC Alpha handguard. I only put two mags through it in about 20 minutes (the first 15 or so rounds to zero and confirm, the other mag and a half just to function test and make sure it worked). No issues with function, but it got HOT! Hot to the point the entire handguard was uncomfortable to touch (not painful, but I wished I had gloves on).
Now, my work rifle is essentially the exact same gun but longer (all BCM, Troy VTAC Alpha rail, but in a 14.5" standard BCM barrel vs 11.5"). I’ve gotten this rifle hot, but not with such a small round-count. ETA: The only other real difference is my work rifle has a YHM Phantom muzzle device, and the SBR has a standard A2, though I can’t really see that causing an issue.
Is this something common with an SBR and nothing to worry about, or is there a potential issue here? I’m a newbie when it comes to the SBR world.
My 11.5 SBR w/BCM barrel and NSR 9 get uncomfortably hot if I put enough rounds through it in a short enough period of time. I put on a pair of gloves and drive on.
Nothing about the config is to blame. Likely the shorter handguard has your support hand a bit closer to the chamber end of the barrel which is closer to the surface of a very skinny handguard.
If you have a chrono, you could spot check some loads to make sure there’s not a pressure issue related to bore diameter. The only other thing I’d say is that some days my guns just SEEM hotter than others.
That handguard is functioning as a heat sink, as designed. The issue is that making the heat sink itself smaller means it’ll get hotter with the same thermal input. Shortening the barrel reduces thermal mass available, same with the handguard, and you have no choice but to hold it right by the gas block, so that’s all trending towards that upper feeling hotter.
Even the bulkier URX unit got noticeably hotter on my 10.5" SBR, I’m going to swap a silver heat deflector panel, and install a panel/hand stop kit just to have some more insulation on the handguard (buying time before being uncomfortably hot more than managing the overall heat picture). I haven’t used the squid grip things enough to comment, but that’s somewhere maybe worth looking - just insulate the paws somewhat.
**Be warned – nerdy answer below:
To add to markm above, your hand is close to the end of the barrel, the gas block/fsb, and you’re running an aluminum rail against the steel barrel and gas block/fsb. Of the two metals: Aluminum has a larger thermal conductivity/higher conduction coefficient and a larger specific heat value than the steel. Basically you have the hot gasses radiating thru the gb/fsb, thru the air gap, and into the aluminum rail. Aluminum transfers heat slowly but releases it quickly (the underside conducting vs topside release to ambient air), so your hand is perceiving the heat transfer thru the rail. The aluminum makes a great heatsink - which in this case is the rail. Clear as mud right? My sbr was an 11.5" bcm bbl with the troy alpha rail - it got warm quickly even with a medium-slow rate of fire.
For the record, I got a low B in Heat Transfer I … so take what I say with a grain of salt.
ETA: I took forever to type my answer; TehLlama beat me to it
One of the reasons standard mil spec short barreled rifles used a heat shield lined polymer handguard, thin barrels raise there temperature much quicker than a heavier profile would by as much as 70* in 30 rounds of rapid but controlled fire up to the 280* range.
All things equal… the only thing that I can think of that would create a real heat difference is a bore diameter issue. Either a tight bore or a bore with a tight spot somewhere in it.
Although highly unlikely with a BCM barrel, no company is perfect. This is likely nothing at all, but if you have a friendly riflesmith in your area, he could run a pin guage down the barrel to check it.