Most of my gun stuff was acquired years ago. About 2008 I built 2 sbr’s so the kids from the ship struggling to pass rifle quals on the VBSS team could shoot something similar away from work. Both on rra lowers. One was a bcm upper and the other was a bushmaster that came with an 11.5” barrel and a 5.5” sleeve pinned on. I just drilled the pin, ground down the front sight gas block, hit it with hi-temp black paint and put on a larue handguard. And about 6 months for the tax stamp.

Fast forward 12 years or so and I decided to put a new barrel on the bcm upper. And I find it has a loose sloppy fit where the barrel pin goes. I tried with 3 different barrels. And since I was curious I took apart the bushmaster. Not loose, a nice snug fit. I’d alway heard the bcm was better than the bushmaster. Granted the bcm looks better with numbered slots on top and a better finish.

I know manufacturing tolerances can cause things like that. But given the difference in receiver to barrel fit I don’t know if a “higher quality” upper with a looser fit is a better choice for a build. Any thoughts?

While I really like BCM in this case I'd go with The better fitting Bushmaster. Back in the day Bushmaster built decent parts.

BCM has never impressed me all that much... the fanboys sure talked em up due to internet fanboyism, but the several examples I have had in my hand have been subpar in a lot of ways.

That being said I have a BCM pencil barrel that has outshot $500 stainless barrels (tighter groups) on numerous occasions ... which makes zero sense .

Pre-2006 Bushmasters were great, but I feel quality started slipping once they were sold.

I bought my last Bushmaster rifle in 2007, but passed on a 2010 model for fit and finish issues.

As far as AR parts go, the "Higher Quality" is measured in fit/finish... the quality of the aluminum used in forging the upper/lower really shouldn't matter much as they are low stress items, but the final milling will make or break a build as far as fit and accuracy goes.

It really boils down to which companies are willing to replace their tooling to keep their tolerances tight, and which play with their tolerances to allow for an extra number of mills per bit.

Is the slop between the barrel extension and the upper receiver?

The inside dimension of the upper receiver should measure 1.000"
The outside dimension of the barrel extension can be made to a spec of .996" to .998" -- with some extensions made to .999", which results in a really tight barrel-to-receiver fit.

Sounds like he’s talking about the barrel index pin slot. BCM over rated, they do typically build a reliable rifle, but they aren’t going to toss parts that are on the large end of spec.

Fit on the barrel index pin slot might be an issue with a FSB barrel, for properly aligning the sights. I don't think it makes a difference with optics or sights mounted on a rail. Once the barrel is properly torqued down via the barrel nut, it probably won't matter. But it does seem better if all parts fit together tightly.

Assuming a upper with an optic, I still go back to using the combo that has the tightest upper receiver to barrel extension fit. In theory, this should produce the best accuracy -- all other things being equal.

I was just surprised at the difference. I can be a little OCD about certain things. This isn’t expected to be a precision rifle by any means.It was the slot on the receiver for the pin. Significantly wider than the slot on the bushmaster or either larue upper I have either. And a pretty loose fit for the barrel extension as well on all the barrels I tried. But it has shot fine til now. Just thought I’d clean things up a bit.