Chamber and throat question

[JCSC]

Im a newbie for checking out chambers, fireformed brass, etc.

It seems that I see a lot of comments about variations in throat depths, lead angles, chamber length, etc.

I started my life as machinist, so the fact that you see so much deviation, bothers me. I understand overlapping tolerances and such in assemblies, but I would think a small bored hole, like a chamber, should be within tenths.

Is this a Grendel theme, or is it commonly seen across the industry. I believe I recall hearing that the 6.5 CM has a wide array of variations as well.

I?m just curious from some of the tenured advisors here.

[Hawk518]

I would like to think the different variations are because there are only about 3 production barrels and the rest are "custom".
Ruger, Howa, and i think CV are the big names. the rest are custom not all cookie cutter. The inexpensive ones turn out alot of barrels before retooling so at the start they are at max spec buy the time the reamer is changed they probably are under min. Also new bullet shapes and lengths change the formula. This is all just a guess but it seems logical to me!!!!

[grayfox]

The Grendel chamber design is standardized and certified by Saami, there's a pdf of the Saami spec.
The throat of this design has a 2-stage angle as it ramps to the grooves, 0.5 deg and then 1.5 deg.
The 264 LBC and so-called "Grendel II" are non-standard and therefore can vary at will, but in general stay close the Saami spec except for using a single angle, more straight-wall approach to the throat area. Throat length can differ as well.

All this being said, as a machinist you can appreciate if someone cuts that chamber with a worn reamer, or doesn't do the right QC and lets a short throated chamber go out the door. Therein, IMO, lie most of the problems; especially so as cut-rate barrel mfrs and high-volume cheaper-barrel mfrs get into the business.

Here is the pdf.

65Grendel_SAAMI_Specification.pdf

[ricsmall]

The biggest problem is the use of a non standard or worn reamer. Chambers in a custom barrel such as precision firearms would typically be held to ~.003” variance. Maybe even closer. Mass produced barrels from one manufacturer to the next will probably vary more, especially taking in to account reamer wear. Reamer manufacturers may have slight differences in exact measurements of the same spec reamer, but should still be within saami spec. A reamer should not change dimensionally for probably 20 chambers, depending on method used, coolant/lubricant use, and quality of barrel steel used. The higher quality 416 barrels generally cut like butter, while say barrel steel comparable to a rem 700 factory gun seems more gummy to me.