Chambering for Straight pull AR 10 type .308 rifle - hard extraction?

[Richard_w]

All,

I am working with an Australian manufacturer of a new manual AR10 type .308 rifle.  

We are having trouble with hard extraction.  

My feeling is that chamber smoothness is going to be one of the main issues.

Others say the headspace is too tight- we are at the lower end of the SAAMI  .308 headspace spec. 

The barrels are also nitrided, and I am wondering if this has an effect om either chamber dimension, or roughness, as well.

I figured that shooters in the UK would have the most experience in such platforms.

So I would be very interested to hear from anyone with experience building or chambering barrels on straight pull AR type rifles- particularly in the larger cartridges like .308.

 

 

[SeaMonkey]

Mark Bradley (Bradders on this forum) is the man to talk to regarding all items AR. I'm sure he'll be along in a minute.

[bradders]

You need to get the chamber as polished/smoothed out as you can and also run it at near max or max headspace, and you also need to get away from the nitriding , at least until you have something reliable, but with regards to the nitriding that barrel will now be u/s as you won’t be able to do anything with it.

A reamer won’t touch it now sadly. All nice ideas but a rethink is needed

What State are you in out of curiosity?

[Richard_w]

Thanks very much for that information Mark, I was hoping you would get in touch.  Nothing like real world experience and knowledge.

I am in Tasmania, most of the work is happening in Victoria though.

What are your thoughts around chamber fluting?

I believe the pump-action Troy PAR AR type rifles are fluted. 

And the POF ReVolt looks to have the their new fluted chamber neck design. 

Have you tried the ReVolt?  Does the fluted chamber neck seem to have any advantage?
 

Yes, there is not much we can do with the existing, nitrided, barrels, we will have to change the specification. 

Do you hand-polish your chambers after the barrels are supplied, or will some barrel manufacturers do that if you specify it?

 

[bradders]

5 hours ago, Richard_w said:

Thanks very much for that information Mark, I was hoping you would get in touch.  Nothing like real world experience and knowledge.

I am in Tasmania, most of the work is happening in Victoria though.

What are your thoughts around chamber fluting?

I believe the pump-action Troy PAR AR type rifles are fluted. 

And the POF ReVolt looks to have the their new fluted chamber neck design. 

Have you tried the ReVolt?  Does the fluted chamber neck seem to have any advantage?
 

Yes, there is not much we can do with the existing, nitrided, barrels, we will have to change the specification. 

Do you hand-polish your chambers after the barrels are supplied, or will some barrel manufacturers do that if you specify it?

 

I turn and chamber all my own barrels and polish the chambers after reaming to #320-#400 and ensure there are no marks in there

I have no experience with chamber fluting, but a good chamber shouldn't be necessary. That said though, it's pretty hot where you are, esp Victoria, so that may be a factor

One thing though, why .308 and not just .223? The smaller cartridge has many benefits over .3908 in a SP and is more than capable

[Richard_w]

Thanks for the further advice Mark.

What technique do you use to polish the chambers?

In hindsight, .223 would gave been a better choice for the first product, I agree.

.308 is popular for pest control in Australia, particularly feral pigs.  This is one of our main markets.  

Looking at some other areas to reduce friction in the action- the bolt face for example.  May try a teflon coating here as well.  

[bradders]

16 minutes ago, Richard_w said:

Thanks for the further advice Mark.

What technique do you use to polish the chambers?

In hindsight, .223 would gave been a better choice for the first product, I agree.

.308 is popular for pest control in Australia, particularly feral pigs.  This is one of our main markets.  

Looking at some other areas to reduce friction in the action- the bolt face for example.  May try a teflon coating here as well.  

Nothing scientific, a split rod with emery cloth run at high speed wetted with a suitable lube to stop it clogging, either on the lathe or with a high speed cordless in a video the bench

Don't go getting carried away Teflon etc, it’s not necessary, just get a working rifle first

Find the lightest/most volumous brass you can to keep pressures down and don’t get too greedy on pressure