6.5 mm PRC build - bolt/barrel clearances

[riflemonkey]

Hello there...

I am a target shooter and not a hunter. I am building a 6.5mm PRC rifle with a custom remington 700 clone short action. I am considering the clearances to make between the bolt and barrel for an m16 style ejector, chrome-moly bolt, and stainless steel barrel. As of now I am considering the following clearances:

1) end of bolt to counter bore - 0.10" (axial)
2) clearance between bolt and counter bore - 0.007" (radial) or 0.014 (diameter)
3) end of bolt lugs to end of barrel face - 0.010" (axial)

Any comments or opinions as to why I should use anything different?
Thanks

Riflemonkey

 

[LVLAaron]

Those are fairly typical. Good to go.

 

[msalm]

With a magnum bolt face and an m16 style extractor, typically the counterbore has to be at least .740” in diameter or .020” clearance all around. Otherwise there won’t be enough clearance for the extractor to snap over the cartridge rim while chambering. Measure the diameter of your bolt nose, then measure the diameter over the extractor as you snap a round into the bolt face. Take the difference of those two measurements, double it and add to the bolt nose diameter. That is the minimum diameter you can have for the counterbore diameter, and likely it will be .010” over that.

 

[LVLAaron]

^ Yes.

 

[Ksracer]

Measure the radial thickness of the extractor hook, double it and add it to the rim diameter. Add .015-.020 to that for your counterbore diameter as a starting point.

 

[Dusty Stevens]

Yep you need more clearance

 

[DaveTooley]

.740"-.745" around here.

 

[msalm]

This has been addressed fully, but as a helpful hint, after cutting your chamber and counterbore, just put the headspace gauge or an empty piece of brass in the chamber and push your bolt into the counterbore to ensure the extractor has enough clearance to snap over the rim. It will save you some time after assembly if it doesn’t work

 

[xr650rRider]

If it's a Badger M16 extractor, the installation instructions specify 0.785" for proper clearance, even though you can get by with less.

https://www.badgerordnance.com/media/binaryanvil/media_library/306-90_91_92_copy.pdf

 

[riflemonkey]

Thanks for the great replies ...

Riflemonkey

 

[DaveTooley]

This has been addressed fully, but as a helpful hint, after cutting your chamber and counterbore, just put the headspace gauge or an empty piece of brass in the chamber and push your bolt into the counterbore to ensure the extractor has enough clearance to snap over the rim. It will save you some time after assembly if it doesn’t work

Or after you Cerakoted the barreled action and have to remove the barrel.

 

[gunsandgunsmithing]

The original Mike Walker Remington 3 Rings of Steel design was intended to have .003-.005 clearance on all surfaces, and it works. Over time, we and they have loosened those tolerances. I've had powder get squished in between the bolt and bbl and never felt a thing. And frankly, what difference does it make when a kernel of n133 for example is what... .030ish. So whether clearance is .003 or .015 isn't gonna matter much in that regard. Where it DOES matter is in how the safety function of the 3 rings of steel design is compromised by excessive unsupported case and clearance. I can tell you of an experience that wasn't my proudest moment but where the phone rang while I was developing data for a wildcat with very fast rifle powders. I had a Pressure Trace hooked up at the time. I'm not saying the numbers were 100% correct, but they were close. It was a ppc case head on a 308 bolt face with a sako extractor. Yep, just about worst case scenario here. Clearances were at .003-.005 throughout and the Pressure Trace went to 123,000psi! The extractor stayed in and the case ruptured at the ejector hole, because the extractor was held captive at that pressure and with those clearances. Add enough pressure and everything becomes a bomb but it spoke volumes to me. The bolt nose did as designed and expanded to fit the counterbore to seeal off the case head and debris as well as captivating the extractor. It couldn't come out far enough to unlock and it stayed put, brass rupturing at the ejector hole.

Lots of smiths use .010 and it has and will generally be fine there. But in a worst case scenario, I want less clearance, as intended. Clearance is just that. After seeing multiple times that a kernel of powder did not lock a gun up....I simply see no reason to loosen those clearances beyond what they were intended to be, just to make my job easier.

That's all, and I'm sure someone will have a different experience to counter mine. So do as you are comfortable doing, but you were informed, either way.

 

[msalm]

I’m sure you understand, but with a mag bolt face and m16 extractor, with a .010” clearance around the bolt nose, you’ll never be able to chamber a round as the extractor won’t snap over the rim. With 308 or 223 bolt face, that’s not an issue as the extractor is machined deeper into the bolt body.

As an aside, if bolt and action are completely blueprinted, everything is concentric to the bolt raceway, I’ve gone as little as .001-2” clearance (diameter, not front clearance) that offered essentially a bushed bolt. No issues with that as long as everything is truly concentric…if it’s off by even .002”, she’ll get a bit tight when camming the bolt closed…then ya gotta open it up a touch more

 

[Dusty Stevens]

.785 is the counterbore measurement for an m16 extractor on a magnum bolt

 

[tobnpr]

If it's a Badger M16 extractor, the installation instructions specify 0.785" for proper clearance, even though you can get by with less.

https://www.badgerordnance.com/media/binaryanvil/media_library/306-90_91_92_copy.pdf

Yep- they do. I used one in a custom PTG bolt. I didn't have the extractor in hand at the time of chambering, but after I bored the counterbore out to their spec and fitted the extractor I found that at no point (as it pivoted) did it extend beyond the face of the bolthead (magnum bolt- .300 WSM) If you want to retain as much of the "ring of steel" as you can, suggest you install/measure the bolthead for clearance needed before cutting the counterbore.

 

[riflemonkey]

Well I went ahead and measured the amount the extractor opens outside of the bolt radius - it actually did not protrude at all. I ended up using the following clearances:
1) end of bolt to counter bore - 0.10" (axial)
2) clearance between bolt and counter bore - 0.020" (radial) or 0.040 (diameter)
3) end of bolt lugs to end of barrel face - 0.010" (axial).
The above image is the final, as built, rifle. Can't wait to shoot it!

Thanks to all who replied to the post!

Riflemonkey