Bolt depth and ejector marks

[WestMOhunter]

I’ve found some great info. here over the years, so finally decided to join. So, hello and thanks! Brief background…. I’ve been building (4), reloading, shooting the 6.5 Grendel round for 6? years, but still plenty to learn.

Anyway, I built another Grendel 6 mo. ago with a Wilson Arms (JSE) 20” barrel. If I could have a do-over I’d take it, but I had a Wilson-barreled Rock River years ago that shot outstanding. Build has “matched” bolt, Superlative GB, Toolcraft carrier, rifle length, etc. blue loctite bedded in billet upper. First outing shot factory 123 SSTs awful. Like first 4-5 we’re 4 moa. Should have sent it back but I made the mistake of Cerakoting it prior to a test run, so I wanted to make it work. Fast-forward and I have a 26.9 gr AR-Comp/123 Nosler CC load that is consistently a 5-shot group in 0.5’-0.75” @100. Shot a couple in the .4s. Barrel is slow at 2450ish, but already getting ejector swipes with factory and handloads, so didn’t bump the charge more. I’ve not seen any other signs of pressure (primers perfect), so just settled for ejector marks. Was playing with seating depth recently and it occurred to me to measure bolt face depth. It’s 0.1405-0.1410. I understood there to be tolerance of .003” (+/-) with the .136” standard, but since told .136” is max depth. That is as much as .005” excessive headspace.

Has anyone encountered this?

Either way, thoughts on this being the culprit of my pressure signs?

Lastly, are there many of those JSE Wilson Arms barrels out there, and how are they shooting?

Thanks!

[WestMOhunter]

Also meant to ask about velocities. I realize there is variability among barrels, but the 20” BHW barrels are doing 2550ish with the same bullet and AR-Comp. Is 2450 not pretty slow for a 20” w/ 123? Thanks.

[VASCAR2]

After using a couple different calipers I’m not sure I would trust the readings on +or- .001”-.002”. Fortunately for me I know a couple machinist with precise micrometers and measuring tools. One thing which bothers me is JSE Surplus sell Wilson 6.5 Grendel barrels chambered for .125” (7.62x39) bolts and barrels chambered for .136 bolts typical for 6.5 Grendel. This is just asking for mismatched parts in my opinion but it is what it is.

Hopefully someone more knowledgable will chime in but as long as the barrel bolt head space probably it should be fine. There have been discussions on how to check head space without gauges on the forum or you could buy or rent head space gauges if your concerned. I’m curious if the Wilson barrels are marked to indicate which bolt is to be used. Black Whole Weaponry/Columbia River Arms sold a lot of 264 LBC barrels chambered to use .125” bolt face depth x39 bolts. BHW weaponry also sold barrels chambered to use .136” 6.5 Grendel bolts. BHW coined the term Type 1 bolt for barrels chambered for .125” x39 bolt and Type 2 for .136” bolt face depth 6.5 Grendel bolt.

If your barrel was chambered to use .125” bolt but was supplied with a .136”+ bolt you probably have excess head space which might push out the shoulder on a 6.5 Grendel case which might be obvious.


Here is a link to JSE showing a Wilson Arms with matched .136 bolt and 20” barrels but they show several barrels with matched .125 bolts.

https://www.jsesurplus.com/wilsonarms206...dbolt.aspx


It is possible the ejector has a stiff spring and will not seat completely which is causing the ejector swipe. If the shoulder set back on fired cases seem in spec I think this might be an ejector issue especially if the bolt face is a couple thousandths deeper. Your loads and firearm might be safe but the ejector might be causing the swipe.

[VASCAR2]

Also meant to ask about velocities. I realize there is variability among barrels, but the 20” BHW barrels are doing 2550ish with the same bullet and AR-Comp. Is 2450 not pretty slow for a 20” w/ 123? Thanks.

I think BHW used some polygonal barrels with 5R twist which tend to produce a little higher velocity compared to an Enfield rifled barrel. The Speer Gold Dot is a bonded core bullet and tends to build pressure faster than a cup and core bullet like the 123 grain SST. My 20” Shilen loaded with 31.0 Grains CFE and CCI 450 primers with 123 grain Nosler CC gave 2526 FPS and I got 2511 FPS with factory 123 grain SST.

I have seen where some barrels tend to produce lower velocities compared to barrels of the same length. It is possible the chamber on the wilson is on the larger size in diameter and produces less velocity.

[WestMOhunter]

Good point on questioning accuracy of my equipment. It’s certainly possible there is error as it’s just a cheap digital caliper. Having said that, it measures very consistently and precision is to .0001. I would be surprised if it’s off by anywhere near .0050” but it’s possible, I suppose.

My BHW barrels are .125 so yes, I’m aware of that concern. I also considered the history with JSE of having Wilsons chambered for both .125 and .136 bolts. Given that I have a .1410 deep bolt, I feel pretty confident the chamber was cut for a .136” bolt. A .1410 bolt with a .125 chambered barrel is gonna be a bigger issue than slight ejector swipe, I assume. I will stick a new in spec bolt in it and see if it heals up.

Thank you for the ideas.

[VASCAR2]

I would measure your bolt overall length as there are bolts with 3.280” length which require a shorter 3.269” length firing pin. Alexander Arms used 3.2810” length bolt from the tail to the face of the lugs to account for the deeper .136” bolt face. Where I’d be concerned with a little deeper bolt face depth was if the bolt overall length was also shorter than 3.2810” as this increases firing pin protrusion. It is possible your bolt is longer which might be why this bolt passed inspection. Say your bolt is 3.285” this would be more apt to keep the firing pin protrusion in spec than a short bolt. Firing pin protrusion minimum spec is .028” with a maximum .036”.


I have several of the Millennium Monster bolts sold on 6.5 Outfitters and these bolts manufacture are very consistent. I always keep a spare bolt so I’d probably buy another bolt as your JSE bolt looks to be out of spec. The deeper bolt face doesn’t appear to have affected the extractor tension or ejection.


https://www.sixfiveoutfitters.com/produc...olt-mk136/

[mtnlvr]

[quote=WestMOhunter]Good point on questioning accuracy of my equipment. It’s certainly possible there is error as it’s just a cheap digital caliper. Having said that, it measures very consistently and precision is to .0001...]
You have a cheap caliper that has resolution to 0.0001"? Most of the calipers I've seen, even quality ones, only display resolution to 0.0005".

[Sinclair]

[quote=WestMOhunter]Good point on questioning accuracy of my equipment. It’s certainly possible there is error as it’s just a cheap digital caliper. Having said that, it measures very consistently and precision is to .0001...]
You have a cheap caliper that has resolution to 0.0001"? Most of the calipers I've seen, even quality ones, only display resolution to 0.0005".

ditto on mtnlvr's comment. I am a retired Tool and Die Maker and was required regularly to work to 4 digits accuracy and some tines 6. Calipers, by their nature, do not provide the "feel" necessary to measure to ten thousands of an inch. Depth micrometers aren't much better, so for safeties sake, I would not put a lot of faith in your measurements. Just my two cents worth.

[WestMOhunter]

[quote=WestMOhunter]Good point on questioning accuracy of my equipment. It’s certainly possible there is error as it’s just a cheap digital caliper. Having said that, it measures very consistently and precision is to .0001...]
You have a cheap caliper that has resolution to 0.0001"? Most of the calipers I've seen, even quality ones, only display resolution to 0.0005".

No. I was thinking “four digits” not 1 ten thousandth, so my mistake. You’re right the precision is to .0005”. Thanks for the correction.

[WestMOhunter]

[quote=mtnlvr]

ditto on mtnlvr's comment. I am a retired Tool and Die Maker and was required regularly to work to 4 digits accuracy and some tines 6. Calipers, by their nature, do not provide the "feel" necessary to measure to ten thousands of an inch. Depth micrometers aren't much better, so for safeties sake, I would not put a lot of faith in your measurements. Just my two cents worth.

Thanks for the info. Yeah, I have been working on the assumption that there is likely error, but not .0050” on a tool that reads to .0005”. But I admittedly don’t know what standards to expect with a digital caliper from an auto parts store. I will definitely keep that in mind. While relative comparisons and assumptions aren’t rock solid, part of the reason I’ve not assumed significant error is because when I measure .125” bolts they’ve been close (.0010-.0015). Anyway, I will quit basing this off of a caliper measurement. I’ll just shoot with a new bolt, and check for case head ejector swipe. Appreciate the help.