6.5 grendel bcg compatible with 7.62x39

[MISportsman]

Everything I read is contradicting. Can I put a 7.62x39 barrel on my 6.5 grendel and by that and the magazine being the only other changes have it function properly and safely?

[BluntForceTrauma]

Probably not. 65G bolts typically have a boltface counterbore depth of .136; in the industry these are called Type 2 bolts. 762x39 bolts typically have a boltface depth of .125; these are referred to as Type 1 bolts.

The two bolts have different headspaces and are thus NOT interchangeable. But this depends on how the barrel is headspaced.

Unless your 762x39 barrel is headspaced for a Type 2 bolt with a .136 boltface depth, then your 65G bolt will NOT work because it will have too large headspace. Too large headspace will slightly shorten your brass life if you reload — not the biggest deal in the world — but in extreme cases it can ultimately cause the case head to stretch so much it weakens and separates from the cartridge body, causing an explosion when you pull the trigger.

[MISportsman]

I'm assuming mine is more than likely a type 2. It is a bear creek complete upper I got on sale a year or 2 ago. Worst case all I should have to do is change the bolt on the bcg to a x39 bolt no right?

[MISportsman]

On the other hand, isn't the grendel and the x39 the same identical case just different neck size?

[VASCAR2]

If you have a Bear Creek Arsenal 7.62x39 upper/barrel you’ll need a 7.62x39 bolt. Some 7.62x39 BCG have longer enhanced firing pins to help ignite hard steel case military primers. The bolt face depth is the difference between 7.62x39 (.125”) bolts and 6.5 Grendel bolts (.136”). As BFT mentioned you have to use the bolt for which your barrel is chambered.

The only 7.62x39 barrels I ever heard of being chambered to use .136” bolt face depth bolts was Colt 7.62x39 AR-15 rifles back in the 90’s.

There can be an issue of firing pin protrusion being out of spec in the AR-15 with AR-15’s. The minimum spec for firing pin protrusion is .028” maximum .036”.

I have never seen a situation where the 6.5 Grendel needed enhanced longer firing pins like is recommended for the 7.62x39 AR-15. There was only one source for steel cased Berdan primed 6.5 Grendel ammunition which was Barnaul which was also sold as Wolf Military Classic 100 grain FMJ.

[Fess]

On the other hand, isn't the grendel and the x39 the same identical case just different neck size?

It is a little more complicated than that. The 6.5 Grendel is based on the 6mm PPC case with the shoulder blown foreword. When compared to the 7.62x39, the rim and base diameters are the same, but the PPC and Grendel have less taper and the shoulder and neck are very different. It is possible to turn 7.62x39 brass into Grendel, but it requires fire-forming to blow out the shoulders and body.
There is actually a reason that the developers of the 6.5 Grendel decided to use a deeper bolt face than the 5.56 NATO. The rim of the Grendel (and 6mm PPC, 6mm ARC and 7.62x39) is thicker than the 5.56 NATO. The bolt is a little deeper to adapt to the thicker rim and make the extractor stronger. Even with the deeper bolts, extractors are the weak link of the Grendel, so pick up an extra one, just in case.