Design of bolts for ARs

[Trooper]

Spoke with Kevin Collins about his design on the Alexander .338 LM sniper rifle. His comments were to the effect that he designed a simple bolt. That too many semi autos have complex bolt designs, too many machining operations.

How does his bolt compare to the bolts in the Lewis Machine Tools (LMT) .308 rifle? Or to the Faxon Firearms and IWI Trevor ARs?

[LRRPF52]

The LMT .308 is pretty straightforward Stoner multi-lug rotating bolt, but AR-10 in size. They work well, and have excellent endurance, but do have many machine operations. I inspected the Faxon Firearms ARAK-21 at SHOT Show, and it uses a Stoner type rotating bolt/cam pin, but in an op-rod based system. One main difference is that it uses radiused locking lugs and a matching barrel extension, reminiscent of the Professional Ordnance Carbon 15's bolt and barrel extension.





This type of bolt design allows easier manufacturing with a common cylindrical tool head for the creation of the lugs, and is inherently stronger, as long as equivalent alloys and heat treating are compared. Knight's Armament uses this on the SR15E3:



The IWI Tavor bolt is kind of a hybrid of the AK and AR15 bolts, but more AK.

[mongoosesnipe]

all you need is a couple of good lugs...

[cory]

The LMT .308 is pretty straightforward Stoner multi-lug rotating bolt, but AR-10 in size. They work well, and have excellent endurance, but do have many machine operations. I inspected the Faxon Firearms ARAK-21 at SHOT Show, and it uses a Stoner type rotating bolt/cam pin, but in an op-rod based system. One main difference is that it uses radiused locking lugs and a matching barrel extension, reminiscent of the Professional Ordnance Carbon 15's bolt and barrel extension.

If only we could get some Grendel bolts designed in a similar fashion, with respect to the geometry of the lugs.

[bwaites]

As I understand, that's a patented design.

[LRRPF52]

Knight's is. I don't know about the Professional Ordnance one (pre-dates Knight's by many years), but there are differences. Professional Ordnance experienced an extremely hostile takeover by Bushmaster, so Bushmaster now owns and manufactures the Carbon 15 series of AR15's.

[cory]

As I understand, that's a patented design.

You only have to change a design by 10%-20% (I don't remember the exact law) to legally have a different design.

I would think that patent pertains to the 5.56 bolt. The Grendel is a modified 7.62x39, so if it's lugs were rounded I'd think it'd be different enough from the 5.56 design.

Nobody on here has the patent number for the aforementioned design, do they?

[SHORT-N-SASSY]

Another example of the radiused bolt lug (scroll down the page, click-on the photo with the caption, "The Shrike 5.56 six-lug bolt is far stronger than that of the M4.") http://www.shootingillustrated.com/i...e-shrike-5-56/

[mongoosesnipe]

changing the number lugs on the bolt would require a complete redesign of bcg and upper to facilitates a different bolt rotation amount to lock and unlock the bolt

going to a rounded lug bolt and extension would be a little stronger and allow the grendel to be pushed a little harder until the next weak point is found which may well be a dangerous one so tread lightly

[Trooper]

Any weight in the gas system, to include the bolt, takes away from weight that can be used to stabilize the barrel (accuracy).

Also the more complex the bolt then the more expensive the rifle. Plus using a five axis CNC is expensive. One cost saving could be to use simpler machine tools and move the bolt from station to station.

[LRRPF52]

The DI Stoner design already eliminates virtually all the weight in the gas system from being in the front of the weapon like early 20th century designs, to the point above the shooter's hand in the bolt carrier group. Adding weight to the barrel with a gas system takes away from accuracy.

If complexity is removed from the bolt, as in the AR15's hybrid bolt/piston, then complexity in machining is only displaced somewhere else, like a conventional piston. The AR15 reduces the number of parts by combining the op-rod style piston and rotating bolt into one piece, and then protects that component from exposure by containing it in the bolt carrier and receiver.

You can spend machine time and QC/QA/TQM on the partially-chromed bolt carrier and bolt, or on a bolt carrier, simpler bolt, op-rod, hard-chromed piston head, and hard-chromed gas block. The weapon with the critical gas system parts more forward will need to undergo a balancing act with barrel profile if you are to get it well-balanced again, a la SCAR.

Looking at some recent bolt gun data from load development with CFE and the 129gr Interbond, we saw that a 20" Grendel bolt action taking advantage of the Grendel case strength appeared to be stable up to 2580fps, which is about 80-110 fps faster than what we would expect in an AR15 with a 24" barrel.

[Russell D]

Correct me if I'm wrong, but I believe that the lugs on AR bolts and barrel extensions are machined using a broach, which is a very cheap and fast way to machine them.

[LRRPF52]

I honestly can't say what all techniques are used for bolts, but there are several different approaches for barrel extensions-no less than 3, to be exact, and each machine shop does it their own way. I do believe broaching is used in at least 2 of them, while the sequence of cutting the feed ramps is either before or after the heat treating.

With the Grendel, you want the feed ramps cut after the heat treat so that you don't warp the part. For beater 5.56 or .223 guns, it doesn't really matter, because the extractor isn't going to encounter warped ID intrusion of the barrel extension teeth like the Grendel will, having a larger diameter case head. Barrel extensions need to have concentricity and uniformity to tolerances that are stricter than the smaller cases.

You can tell right away on a Grendel if one of those cheaper barrel extensions was used, because the outside of the extractor will be scored adjacent to the reinforcement spine/lug where it is binding against the barrel extension teeth.

[Tedward]

So who makes those Grendel Barrels and uses cheap extensions??

[Nukes]

So who makes those Grendel Barrels and uses cheap extensions??

Who makes those Grendel Barrels and uses cheap extensions?