BCM thermo fit upper fun

[Mike98632]

if it's available, LN2 works pretty well at chilling things down. otherwise a deep freeze makes the barrel cold enough it hurts to hang on to it with bare hands.
-tdbru

I've heard dry ice -109 F. Don't handle it or the barrel bare handed. I would also consult with someone who knows what it might do to the barrel. I don't think anything but I'm no expert

[teppou]

Oh I should also mention that my factory criterion barrel was a very sloppy fit inside the BCM upper and slid right in. I couldn't shoot anything under 1 inch with that barrel.

Hmm, I had the same sloppy fit on a Criterion and BCM, with similar results... Makes me wonder if I should pull it and start over.

[Laman]

My Monster 20" 6 ARC barrel was a tight fit for the BCM upper. A propane torch from a distance and a couple of taps with a wooden mallet and it was in. If there is any wiggle I don't have any instrument that would measure it. I hope to be able to try it out next week.

[drewthebrave]

I've used a handful of BCM uppers on builds, and it seems like every fitting is a little different. My LaRue PredatAR, Bergara SPR, and Alexander Arms barrels took a heat gun and a few light taps of a mallet to get set. A Wilson Combat SR barrel was juuust able to slide in without any thermal fitting needed at all, so I bedded with some Loc-tite to take up the very minimal slack. FWIW, that same WC barrel wiggled like crazy in a standard upper receiver.

The toughest assembly was an Anderson Mfg Grendel barrel that I couldn't get past 1/8" into the receiver with heat alone. So I put the barrel in the freezer overnight. I heated the receiver over my stove (right next to the fridge) to get everything together while the temp differential was most extreme. It got the barrel about 75% of the way in to the receiver before it got stuck. Each part was literally just seconds away from their heat/cold source when it stopped moving. I hammered it the rest of the way with a mallet before the fit had time to get much tighter. Despite the tight fit, that particular barrel wasn't a great shooter, whereas all the others were solid performers.

[Old Bob]

I've used a handful of BCM uppers on builds, and it seems like every fitting is a little different. My LaRue PredatAR, Bergara SPR, and Alexander Arms barrels took a heat gun and a few light taps of a mallet to get set. A Wilson Combat SR barrel was juuust able to slide in without any thermal fitting needed at all, so I bedded with some Loc-tite to take up the very minimal slack. FWIW, that same WC barrel wiggled like crazy in a standard upper receiver.

The toughest assembly was an Anderson Mfg Grendel barrel that I couldn't get past 1/8" into the receiver with heat alone. So I put the barrel in the freezer overnight. I heated the receiver over my stove (right next to the fridge) to get everything together while the temp differential was most extreme. It got the barrel about 75% of the way in to the receiver before it got stuck. Each part was literally just seconds away from their heat/cold source when it stopped moving. I hammered it the rest of the way with a mallet before the fit had time to get much tighter. Despite the tight fit, that particular barrel wasn't a great shooter, whereas all the others were solid performers.

When it gets this close (75%) use a barrel nut & barrel nut wrench to pull the barrel the rest of the way into to upper. No need to smack it with a mallet.

[mv1arms]

I shot a workup of 8208 XBR from 26.8 to 28.5 with 120gr smks. Didn't notice any overall group improvements from my workup of LVR in the old aero upper. Hard to compare between the two but it was the same bullet. Shot some sub moa groups and some not as good groups. All groups were 5 shots. Standard deviations were single digits for the good groups. Mild pressure signs at 28.5 grains.

Edit: will post the pic when I'm back on my computer.

[mv1arms]

CollageMaker_20210424_175718714_resize_35.jpg

XBR is with the BCM
LVR is with the aero that I bedded with loctite.

I see nothing magical about the BCM upper but obviously this isn't apples to apples with different powders.

edit: trying to get the photo to load.

[StoneHendge]

I see nothing magical about the BCM upper

Heresey!