Design of bolts for ARs

[LRRPF52]

Not AA.

[woohoo]

Most barrel producers, the ones that make thousands upon thousands of AR15 barrels use AO barrel extensions. One reason the extractors contact the extension lugs is the rim of the Grendel is larger so the extractor must move out further to snap over the rim. The ID of the barrel extension which is what the extractor would contact is a spec shown on the drawings. That spec was made for the 5.56, the extractors clear easily. The extensions are broach cut and then tempered mildly, they are not hard... not near 60 anyway. They are either in spec or they are not. Since the Grendel was shoehorned into a AR15 if there were issues with contacting, the ID should have been enlarged and extensions designed for use on Grendel barrels only.
There are some extensions made by a smaller company that are not good. The depth is off which throws off the headspace and the feed ramps are off sometimes, the part must be moving in the fixture.
There are also some extractors that are thicker and stronger than a standard 5.56 extractor. The relief cut on the back is not there, they can contact when others will not.

[LRRPF52]

This is one of the reasons why AA has their extensions made to critical dimensions, and has the heat-treat done at a certain stage in the process as to not warp the part.

If AA was to have used a different upper, different extension, etc., it would have placed the Grendel in an even less accessible manufacturing posture, and these parts would be bottle-necks in the supply chain.

The decision to use as many AR15 parts makes the cartridge more accessible to consumers. Keeping the pressure at 50,000 psi mitigates the issues one would run into if they were exceeded. In the end, the BC and SD does more than enough to make up for initial muzzle velocity, which is fine where it is.

There will always be those who want more, more, and more muzzle velocity. The current system exceeds the marksmanship capabilities at distance of most adults, while serving youths and recoil-sensitive shooters so well that they can literally shoot for days on end and not experience fatigue.

More ability to practice equals more ability to hit what you aim at, and you get stories like these:

I finally talked my wife in to trying hunting. She is a good athlete and has become a good shot. When she first married me 12 years ago she was from San Diego and somewhat ambivalent to shooting or hunting. This season she drew a Bull Elk tag in Idaho close to where we live and camp (public land). She has always shot my AR's better than anything else due to the adjustability of the stock she being 5'3". I thought this would be an good time to build a 6.5 Grendel for her to shoot. Though many will say it is too light I would rather have her shoot something she is not afraid of recoil wise and hit where she is aiming. We woke at 3:30 AM and mountain biked in the dark into the area we had hunted previously seeing elk but not getting a shot. I had walked about 400 yards away when I heard the single shot. This bull did not take one step and dropped after being hit with a handloaded 125gr Nosler Partition. I know it is not the monsters I see some others post but to her this was a huge accomplishment that she will never forget. We are having friends over tomorrow for an Elk roast Sunday dinner.

http://www.ar15.com/archive/topic.html?b=10&f=2&t=629779

[woohoo]

This is one of the reasons why AA has their extensions made to critical dimensions, and has the heat-treat done at a certain stage in the process as to not warp the part.

If AA was to have used a different upper, different extension, etc., it would have placed the Grendel in an even less accessible manufacturing posture, and these parts would be bottle-necks in the supply chain.

The decision to use as many AR15 parts makes the cartridge more accessible to consumers. Keeping the pressure at 50,000 psi mitigates the issues one would run into if they were exceeded. In the end, the BC and SD does more than enough to make up for initial muzzle velocity, which is fine where it is.

There will always be those who want more, more, and more muzzle velocity. The current system exceeds the marksmanship capabilities at distance of most adults, while serving youths and recoil-sensitive shooters so well that they can literally shoot for days on end and not experience fatigue.

More ability to practice equals more ability to hit what you aim at, and you get stories like these:





http://www.ar15.com/archive/topic.ht...0&f=2&t=629779

I'd like to see someone heat treat and then broach a part...from the other side of the glass. Seems to me they would just make the ID larger and then not need to consider broaching a hard part. Putting a larger size bit in the tool holder doesn't take any more time than a smaller one. Or they could just redesign the broach to remove more material. Gas carburizing a symmetrical part doesn't warp anything.
We use AO mil-spec 5.56 extensions and have never had an issue with extractors hitting not even the thick machined extractors.
What is the ID of these specially made Grendel extensions?

[LRRPF52]

From my understanding, the extensions are the same dimensions, but held to tighter tolerances so there is no intrusion into the ID with the teeth.

I don't remember what method was used to cut the ramps. I talked with Bill about it, and I know it's something he has worked out to avoid the extractor binding issues.

I also learned something about extension dimensions that I didn't know was there all along. It helps to have the TDP when navigating parts sourcing.

[SHORT-N-SASSY]

. . . If AA was to have used a different upper, different extension, etc., it would have placed the Grendel in an even less accessible manufacturing posture, and these parts would be bottle-necks in the supply chain.

The decision to use as many AR15 parts makes the cartridge more accessible to consumers. Keeping the pressure at 50,000 psi mitigates the issues one would run into if they were exceeded. In the end, the BC and SD does more than enough to make up for initial muzzle velocity, which is fine where it is. . . .

First things, first: Nice Elk! One shot, one kill --- a responsible hunter.

LRRPF52, re your statement, above, you'd make a great car saleman. Personally, I prefer the model you were selling in the "Grendel Evolution" Thread, Reply #44, ". . . What needs to happen is for a new gun to be built around the cartridge, rather than trying to stuff .260 performance into a rifle that was literally built around the 50,000 psi .222 Remington. . . ." --- not to mention the increased Bolt Thrust resulting from the larger case diameter of the 6.5 Grendel cartridge.

[woohoo]

Are you saying the feed ramps have something to do with the extractor binding? The feed ramps are at 6 O'clock and the extractors pass through 45 degrees off TDC or 1:30. How do they effect anything to do with the extractor?

I have a feeling you have a big 55 gal drum with AA stenciled on the side as a coffee table. A straw stuck in there and your computer on top. Possibly a red phone with a direct line to the bat cave. Long term exposure to certain elements can cause heath issues. lol

[LRRPF52]

That was under the premise where people were trying to get .260 performance out of the AR15. At the end of the day, there are significant trade-offs.

* More muzzle velocity = more case volume necessary, as you can only do so much with pressure, which has diminishing returns in performance, with increasing returns in penalties that aren't worth it.

* The 6.5 Grendel as it is finds a balancing place between performance (name me one factory chambering that out-classes it in the AR15), and ability to be manufactured

* As soon as you expect more performance from the cartridge that very few have even pushed to the limits currently, you sacrifice the benefit of parts commonality.

I'll give you a personal example. I have always considered my little 16" Grendel carbine a 700yd maximum effective range system with the 123gr Hornady A-MAX, 123gr SMK, or 123gr Scenar, for hitting man-sized targets. Only because of a dare, I realized that I was short-changing it by 500yds in summer conditions here in the mountains. I found that I was able to predictably and consistently hit such a target at 1200yds, as fast as I could break the trigger.

I already knew that I smoked 5.56 77gr at 600yds, but didn't realize I was staying supersonic out to over 1300yds until a situation led me to stretch the legs way farther than I normally would have. Granted, at sea level, I would be limited to 1050yds at 59 F. That's with the factory Hornady 123gr A-MAX from a 16" carbine, going 2470fps.

What kind of performance are you needing again, and what are you willing to sacrifice to get it?

[SHORT-N-SASSY]

. . . * The 6.5 Grendel as it is finds a balancing place between performance . . .

Why is the word, "Balance," used so often on the 6.5 Grendel Forum? It's hardly seen on others. Isn't it just possible that Colt Competition Rifle and others prefer not to do the balancing act?

LRRPF52, you should know that you've earned my respect long before I became a 6.5 Grendel Forum Member. Indeed, most of what you advocate makes good sense --- but, it also keeps us from seeing over the horizon!

[Tedward]

Are you saying the feed ramps have something to do with the extractor binding? The feed ramps are at 6 O'clock and the extractors pass through 45 degrees off TDC or 1:30. How do they effect anything to do with the extractor?

I have a feeling you have a big 55 gal drum with AA stenciled on the side as a coffee table. A straw stuck in there and your computer on top. Possibly a red phone with a direct line to the bat cave. Long term exposure to certain elements can cause heath issues. lol

Woohoo, I think the answer is being avoided or can not support the comment that was made. Thanks for your input.

[LRRPF52]

Are you saying the feed ramps have something to do with the extractor binding? The feed ramps are at 6 O'clock and the extractors pass through 45 degrees off TDC or 1:30. How do they effect anything to do with the extractor?

I have a feeling you have a big 55 gal drum with AA stenciled on the side as a coffee table. A straw stuck in there and your computer on top. Possibly a red phone with a direct line to the bat cave. Long term exposure to certain elements can cause heath issues. lol

Harrison, I've been dealing with AR15's since the 1980's. You know that I know where the feed ramps are. The broaching of the feed ramps before heat-treating can warp the part, because you don't have equal distribution of heat throughout the mass, now that two large chunks are removed from it.

Since you're my elder, and I was taught to respect my elders, I'll refrain from the personal attacks. Either there is merit to my statements, or there isn't. If I am missing something technical that you care to enlighten us on, please feel free to. I suspect that you would not speak to me in this manner if we were face to face.

I make it a point to treat other forum members with the adult respect that any reasonable person would anticipate in gentlemanly discourse. Anyone on the forum can attest to that.

[LRRPF52]

Woohoo, I think the answer is being avoided or can not stand up to the comment that was made. Thanks for your input.

I can do my speaking and thinking for myself.

[Tedward]

I can do my speaking and thinking for myself.

thanks LRRPF52, I just wanted the answer to who makes cheap extensions as you said. Not poking.


The topic went from inadequate parts to single shot takes Elk story.

[cory]

...* As soon as you expect more performance from the cartridge that very few have even pushed to the limits currently, you sacrifice the benefit of parts commonality...

An AR12, at least as I envision it would sacrifice little in parts commonality with what's on the market for ARs now.

There's no reason an AR12 shouldn't utilize the same LPK. Maybe, just maybe it'll need a longer mag release latch.

I see no reason it shouldn't utilize the AR15 carrier. Therefore it should utilize the AR15 charging handle, unless you want to redesign it to utilize a slick side charging handle.

It should be able to utilize AR15 rifle and carbine buffer system. However, I think it's likely that it would greatly benefit from an VLTOR midlength buffer system or something similar.

Essentially a weapon could be designed around the Grendel (& the 6.8 to make it a more profitable investment) that would only require a specifically built lower, upper, bolt, & extension. Oh and magazines.

[LRRPF52]

The Elk hunting story illustrates what happens when you are able to practice with a low-recoiling cartridge that has good projectile selection with bullets that have high sectional densities. In this case, the guy ventured off to find an elk to shoot early in the morning, only to hear his wife make a shot, then he finds that she DRT'd it in his absence.

There are some people who are suggesting that we need more and more muzzle velocity, when the currently-available system is able to harvest game as documented above. These performance demands require a different bolt and extension to be made, so that one can chase the .260 Remington out of the AR15 platform, which you will never reach unless you increase case capacity and pressure, since the .260 Rem is a 60,000 psi SAAMI-rated cartridge.

Even my heavy 22" target-barreled .260 Rem AR10 can be stiff on the shoulder. I wonder what other parts would start to give if I stuffed that much pressure into an AR15, even if the bolt and extension were able to handle it. There are some engineering considerations that need to be accounted for when asking for .260 Rem performance in the AR15 receiver set. The DPMS LR-308 bolt isn't really adequate at dealing with .260 Rem pressures, since it has a rather large firing pin hole.