Gunsmithing Effect of Shoulder Diameter on Reamer/Chamber

[samb300]

Question that I can't find much information on, with background info first...

I'm thinking about a 6 BRA barrel for next season of PRS-style match shooting (definitely NOT bench-rest style), so I've asked for some reamer prints from a few reputable makers. Other than the neck diameter and freebore differences (which I understand), I'm struggling to understand what effect - if any - the shoulder diameter will have. The three makers I contacted have significantly different shoulder dimensions.

My build will be done under the assumption that I will buy a new reamer along with a large lot of virgin brass, and possibly have a custom FL sizing die made. I'm planning on running no-neck-turn brass, so I'm thinking of a neck diameter of ~0.272" and a freebore of ~0.104" to shoot 105gr bullets.

Here is the information from the three different prints:
1) Base = 0.4714", Shoulder = 0.461", Neck = 0.272"
2) Base = 0.4708", Shoulder = 0.463", Neck = 0.271"
3) Base = 0.4710", Shoulder = 0.466", Neck = 0.267"

So...smaller shoulder diameter means more case taper, and larger shoulder diameter means less case taper. Ignoring the neck diameter differences, is there any reason to go with one shoulder diameter over another for PRS-style shooting?

Thanks,

Sam

 

[Keith Johns]

Shoot something else.

 

[samb300]

Shoot something else.

Umm...thanks? Not sure what to make of your comment, if you have some insight to give or just don't like 6 BRA, it's not clear.

 

[idahoorion]

Samb300,

many dashers are in the 460 - 461 area. if you are considering cutting down a dasher size die for your bra, you'll likely need to take that into consideration. sizing a shoulder down .005 or 006. in the case of the .466 doesn't sound so good. I know many cite a rule of thumb for case taper when you are wildcatting but I do not know it. I'd bother the guys on accurate shooting in the 6br forum to get the skinny if it was me.

Idahoorion

 

[Dlwheel]

Question that I can't find much information on, with background info first...

I'm thinking about a 6 BRA barrel for next season of PRS-style match shooting (definitely NOT bench-rest style), so I've asked for some reamer prints from a few reputable makers. Other than the neck diameter and freebore differences (which I understand), I'm struggling to understand what effect - if any - the shoulder diameter will have. The three makers I contacted have significantly different shoulder dimensions.

My build will be done under the assumption that I will buy a new reamer along with a large lot of virgin brass, and possibly have a custom FL sizing die made. I'm planning on running no-neck-turn brass, so I'm thinking of a neck diameter of ~0.272" and a freebore of ~0.104" to shoot 105gr bullets.

Here is the information from the three different prints:
1) Base = 0.4714", Shoulder = 0.461", Neck = 0.272"
2) Base = 0.4708", Shoulder = 0.463", Neck = 0.271"
3) Base = 0.4710", Shoulder = 0.466", Neck = 0.267"

So...smaller shoulder diameter means more case taper, and larger shoulder diameter means less case taper. Ignoring the neck diameter differences, is there any reason to go with one shoulder diameter over another for PRS-style shooting?

Thanks,

Sam

Many are just using the Alex Wheeler reamer design. Your #1 above look like his spec, except with the .272 neck. Many are shooting that spec. I’d just stick with that and be done with it. But be sure and get a die that matches, FWIW.
BTW, JGS has the .267 neck reamer in stock. I have the no-turn reamer in the queue.

 

[samb300]

Many are just using the Alex Wheeler reamer design. Your #1 above look like his spec, except with the .272 neck. Many are shooting that spec. I’d just stick with that and be done with it. But be sure and get a die that matches, FWIW.
BTW, JGS has the .267 neck reamer in stock. I have the no-turn reamer in the queue.

You are correct. That default reamer is with a 0.268" neck which won't work for no-turn Lapua brass, but it can be had with a larger diameter.

Based on my math, the Wheeler reamer has the same case taper as a standard 6.5x47L and 308 Win. reamer. The other two options have much less, with option #3 having half the taper.

 

[Keith Johns]

Umm...thanks? Not sure what to make of your comment, if you have some insight to give or just don't like 6 BRA, it's not clear.

Sorry I know it doesn’t help with the 6bra.
But with all the great cartridges in existence that have readily available brass and components, all proven reliable by now, it’s not a bad suggestion.

 

[LRI]

Question that I can't find much information on, with background info first...

I'm thinking about a 6 BRA barrel for next season of PRS-style match shooting (definitely NOT bench-rest style), so I've asked for some reamer prints from a few reputable makers. Other than the neck diameter and freebore differences (which I understand), I'm struggling to understand what effect - if any - the shoulder diameter will have. The three makers I contacted have significantly different shoulder dimensions.

My build will be done under the assumption that I will buy a new reamer along with a large lot of virgin brass, and possibly have a custom FL sizing die made. I'm planning on running no-neck-turn brass, so I'm thinking of a neck diameter of ~0.272" and a freebore of ~0.104" to shoot 105gr bullets.

Here is the information from the three different prints:
1) Base = 0.4714", Shoulder = 0.461", Neck = 0.272"
2) Base = 0.4708", Shoulder = 0.463", Neck = 0.271"
3) Base = 0.4710", Shoulder = 0.466", Neck = 0.267"

So...smaller shoulder diameter means more case taper, and larger shoulder diameter means less case taper. Ignoring the neck diameter differences, is there any reason to go with one shoulder diameter over another for PRS-style shooting?

Thanks,

Sam

Hi Sam, maybe this will help some.

The base OD/ID of a cartridge/chamber is often varied based on the manufacturer of both the gun and brass being used. The general rule of thumb that I have adopted is that most of the domestic stuff made by big name, high volume companies, tends to favor the smaller side of the dimensional allowance. Europe tends to run bigger.

An example. The 308 Winchester SAAMI's out at .4712" yet Winchester produced brass typically runs around .463" -ish. One will immediately see this when examining fired cases in typical production chambers. The web region of the cartridge often produces an oddly shaped swollen ring feature that is kinda wavy across the periphery. I have my own pet name for this: "Muffin Top" as its about like a fat girl in tight pants.

Not exactly flattering, but accurate none the less.

Now, the gun will likely shoot like this and still produce sterling accuracy, assuming all other things are done well of course. The issue becomes your mental anguish at the reloading bench and the likelyhood of diminished service life. Dies are going to encourage this stuff to squish back to something more appropriate looking. Brass is not an accordian so it will only tolerate it so long. Annealing case webs is kinda suicidal so it too is out.

What to do?

As you see, chambers can be varied simply by selecting a tool appropriate for the job. More however does play into this. A PRS/NRL "Gamer Gun" has to do one thing very well before anything else; It's gotta go bang and that only happens when ammo gets into the chamber. My point is its not bench rest shot in pristine environments under very controlled circumstance. In fact, the polar opposite often happens.

So, build the gun to run cold, hot, wet, funky, etc.

With that, select a reamer that allows for minimal case growth in the web. Lapua Brass in .308 typically runs at about .4675 to .468 in diameter over the web. With a .4712 reamer that seems to work well. I would look to something similar for clearance in your BRA, BRX, whatever. The rule really doesn't change all that much.

The ghist in all this is to get a sample lot of brass, measure some pieces, then pick your tool. Keep your shoulder bump to just the minimum required and then lay the hate.

Hope this helped.

C.

 

[samb300]

@LongRifles Inc. thanks so much for the detailed response. I think step #1 for me will be to buy a large lot of 6 BR brass to get some measurements (and a sanity check from all the internet info out there). Seems like if I’m going to buy my own reamer I might as well take the time to do it right, and keep brass consistent for a few barrels worth of shooting.

As of right now the 0.4714” base and 0.461” shoulder seems like the ticket, with something like a 0.272” dia neck. I’ve been running a 4-5 thou neck clearance in my 6.5x47L with no ill effects, so I have no desire to go tighter than that for PRS shooting.

Chad - your response is one of the reasons I’ve been using LRI for my work over the past couple years. I’ll be sending you a blank for another 6.5x47L AX barrel soon, and when I get the gear in place for a 6 BRA barrel it’ll be headed your way as well.

 

[Dlwheel]

@LongRifles Inc. thanks so much for the detailed response. I think step #1 for me will be to buy a large lot of 6 BR brass to get some measurements (and a sanity check from all the internet info out there). Seems like if I’m going to buy my own reamer I might as well take the time to do it right, and keep brass consistent for a few barrels worth of shooting.

As of right now the 0.4714” base and 0.461” shoulder seems like the ticket, with something like a 0.272” dia neck. I’ve been running a 4-5 thou neck clearance in my 6.5x47L with no ill effects, so I have no desire to go tighter than that for PRS shooting.

Chad - your response is one of the reasons I’ve been using LRI for my work over the past couple years. I’ll be sending you a blank for another 6.5x47L AX barrel soon, and when I get the gear in place for a 6 BRA barrel it’ll be headed your way as well.

FWIW, I did confirm with JGS that Alex changed (09.21.17) the shoulder to .460 to match the BR Lapua Brass and most 'off the shelf' sizing dies.
-dlw

 

[Sheldon N]

I'll also mention that talking to John Whidden, the Whidden 6BRA die was made based directly off the Wheeler 6BRA reamer print. It's as though you were ordering a custom die from them if you are using that chamber. Harrells and Forster dies also closely match the Wheeler print dimensions. Forster is a bit tighter at the base than Whidden, and Harrells can be had in varying degrees of base diameter depending on whether you get a D3 or D4, etc. So the main advantage of going with a .460 shoulder would be not having to buy a custom sizing die.