Have a TBAC338BA in transfer, and acquired two 338 brakes to use on my 338 LP and 300 RUM. I noticed the one the 'smith put on the 338 has flats behind the shoulder while the brake on the 300 RUM is like all the others I've seen, ending behind the shoulder.

Personally I don't give a shit either way, I'm just curious because I can't find any images of one with the flats.

Are the flats part of the brake (did you buy them new)?

it looks like it could be a thread adapter???

looks like an AAC brake..  Other than that I;m unsure.

It's part of the brake, both of the brakes are marked for Thunderbeast and both have the same thread dimensions.

It looks like the extra flat on the 338LP is screaming, "HOLD ON TIGHT!"

I would contact Zak through Thunderbeast.  I've never seen a brake with flats like that. It's either a one off, old design,  or some gun smith pieced together special.

This. Try at the TBAC Forum

Turns out silencershop screwed up and shipped me an MRAD 3/4-20 when I ordered two 3/4-24s. Figures.

Now you have a reason to buy a MRAD!  Congrats.

So did your smith end up threading the barrel to 3/4-20 instead of 3/4-24?

 

They were shipped to him a few weeks apart, and I was out of the country for a few months semi-incommunicado during that time. The 338 is a new gun and the 300 RUM was rebarreled and reworked, we had discussed different thread pitches and muzzle devices but no solid decision. I told him I would decide and have something shipped to him, so he didn't thread it until the muzzle device was in hand. I should have clarified what was coming, but he's a friend and I've always had at least one thing in a constant state of construction at the shop for 15+ years, so he's used to my occasional unusual instructions.

It's really no big deal to me ulitmately, as the gun is a tackdriver. I was just really curious as to what was happening with this brake.