Type II bolt spec and questions...

[Sovereign77]

Looking for some more detail on type II bolt specification. I have heard folks say that the spec is 0.136 +0.00/-0.003. Is this true, and where is the data to support? What's interesting is that a type II bolt (which should be .136) may be more like 0.138 or more, and if they use it during chambering to set headspace, it could lead to a condition in which A. it headspace gauges fine and doesn't close on a no go, AND B. leads to unsupported case area if a generous feed cone is present, leading to larger than normal case head swelling and what appears as a belt near the case webbing after sizing. Tell me if I'm off the mark with these thoughts...

[BluntForceTrauma]

A 5.56 bolt counterbore depth is 0.122 +0.00/-0.003 for a max depth of 0.125. However, since there's no established military spec for Grendel bolts, you can't make a direct comparison. All the Grendel bolts I've dealt with have been 0.136 counterbore depth, or a very few at 0.1365. Don't know what tolerance bolt manufacturers are giving themselves, but practically speaking, they're manufacturing them pretty dead on.

I think any variance comes with how barrel makers are setting the tolerance range of their headspacing.

[LRRPF52]

Looking for some more detail on type II bolt specification. I have heard folks say that the spec is 0.136 +0.00/-0.003. Is this true, and where is the data to support? What's interesting is that a type II bolt (which should be .136) may be more like 0.138 or more, and if they use it during chambering to set headspace, it could lead to a condition in which A. it headspace gauges fine and doesn't close on a no go, AND B. leads to unsupported case area if a generous feed cone is present, leading to larger than normal case head swelling and what appears as a belt near the case webbing after sizing. Tell me if I'm off the mark with these thoughts...

The term “Type II” came from Black Hole Weaponry for their own internal delineation between .136” and .125” bolts, many years after Alexander Arms introduced 6.5 Grendel to the market.

History is this:


Colt developed the 7.62x39 Sporter AR-15s from the late 1980s-early 1990s, which was a funded project by an unnamed entity.



Bill A. researched Colt’s work on that effort when developing the .50 Beowulf bolts with .136” recess, (based on a rebated cartridge using similar case head dimensions to 7.62x39).



6.5 Grendel bolts spec’d and made by Alexander Arms in the early 2000s-present. Small dimensional tweaks were made to the exctractor shelf height, extractor pin height, and bolt length to accommodate standard AR-15 firing pins eventually. Face depth was always set at .136” with up to -.003” allowable (.133”), no + depth allowable.

AA licensed bolts and reamers out to companies that wanted to make rifles and barrels according to the specs, before SAAMI was approved.

Cheap brands like Model 1 Sales and some other gun show-grade parts retailers figured they could just use some of the incorrectly-dimensioned, garbage metallurgy 7.62x39 bolts on the market (All of which had ~.125” bolt faces).



Several shops looking to get into 6.5 Grendel (thinking the important TDP aspects could be avoided), sourced some of these garbage bolts, and headspaced chambers to them.

When BHW came along (they made decent-to-higher quality polygonal rifled pipes), they had been accustomed to using after-market 7.62x39 bolts for 7.62x39 barrels.

They started chambering .292” neck 6.5 non-SAAMI chambers that would shoot most 6.5 Grendel factory ammo, headspaced to the same .125” depth 7.62x39 bolts.

BHW labeled these “Type I”, after hearing that "AA had just changed the bolt face depth to make it harder for other manufacturers to get into 6.5 Grendel.”, rather than looking at the mechanical and firearms engineering merits to the deeper bolt face (extractor lip strength).

When BHW customers asked for actual Grendel bolt-compatible chambers with a .136” bolt face depths, BHW labeled these “Type II” to help distinguish between the two so customers wouldn’t miss-match.

This is where the whole “Type II” misnomer came from. It isn’t a real thing, but it was an easy way for many in the industry to refer to when they really meant 6.5 Grendel bolts with .136” face depth.

A 6.5 Grendel bolt has many important dimensional and metallurgical call-outs that make it what it is.

Almost every company makes .136” compliant bolts and chambers nowadays. Biggest things to look for are:

How long has the company been around and do they know the differences?

AA, PF, JP, LaRue, Maxim, and the Mk.136 bolts and barrels are known quantities in terms of dimensional adherence and durability. I’m sure I’m forgetting some others.

[Sovereign77]

LRRPF52, this is really a great comprehensive background on the bolt face difference. The reason why I had asked is I am on the next step with Shilen on my deep bolt/feed cone issue. I have 3 barrels that were pulled for whatever reason (poor accuracy and corrosion issues-may be my fault) Either way, each one was installed using headspace gages go, no-go, and field. Every barrel installed did not close on the no-go gage. Early in my investigation I sized and cut away 2 cases and later used a factory starline case. I never thought to examine how much support each case has. I used a borescope to meet the end of the cut a way case and the end of the feed cone, so purely visual and not specific measurements. Surprisingly in each case (perhaps with the exception of hornady with more meat and narrower case base) the case web is a bit beyond the end of the supported section of the chamber. The Shilen had the most case exposure of all of them. I will post some images in the next post with what I mean.

[Sovereign77]

Shilen with hornady case (closes with some hand force)
[ATTACH=CONFIG]19259[/ATTACH]
Shilen with Lapua case (closes with some hand force)
[ATTACH=CONFIG]19260[/ATTACH]
Shilen with Factory Starline (closes with no force)
[ATTACH=CONFIG]19261[/ATTACH]