.50 Beowulf necked down to 6.5?

[LRRPF52]

The other problem using the big bore AR15 parent cases is that they are engineered with FoS for very low working pressures compared to center-fire rifle cartridges.

Think 35ksi region for working pressure, vs 50-65ksi. That has significant considerations for the brass metallurgy and yield. What's the top end on the Beowulf brass?

As you neck down the gasket, the pressure increases due to choking the funnel for gas expansion.

The whole idea behind a necked down big bore in the AR15 is to stay in the AR15 frame size without having to go to the larger AR10 frame.

I saw a build recently where a guy wanted to be as lightweight as possible with a .308 Winchester AR10-sized 16" blaster.

He used a 2A Xanthos receiver set and handguard, lightweight 16" .308 pencil barrel, a lightweight BCM stock, and still ended up over 10lbs once he put glass on it (Leupold in Leupold mount) and loaded magazine.

Now you know why my large frame components sit in pieces.

To do a short, fat 6.5mm cartridge in the AR15 that exceeds Grendel case capacity, you need some serious re-designs to the critical components of the AR.

The next case size up from Grendel is the 6mm BR, which has a .473" head diameter like a .308 Win.

Next up is the .30 Remington AR.

Since the engineering and component development has been done for the 30 RAR (upper receiver, bolt, barrel extension, mags, barrel nut), I really feel this is your best bet to chase .260 Rem performance from the AR15 frame, but the rifle will not behave very well in the recoil department.

Best bet is to get a 30 RAR upper receiver and mags, as much ammo as possible while it's in production, and have a competent smith work with you on a reamer and new barrel.

The 30 RAR has only a slightly longer case, so you will still have a good selection of 6.5mm projectiles for it if one were to do a 6.5-30 RAR.



Grendel in the middle, 30 RAR on the far right






In the end, you go through a lot of trouble to get an extra 150-200fps.

Working pressure is still limited to 55ksi because of bolt lug length, even with the larger diameter bolt and extension, like on the 30 RAR.