Relationship between CBTO and freebore demension

[Boisedoc]

I am having a smith build a 300 WSM. I plan to shoot 212 eldx bullets. He has a SAAMI reamer. I've done a lot of research and the consensus is a .200 -.220 freebore. The SAAMI spec is .188. He said I could load a dummy round for him and he can ream based on that OR I hear there are ways of ordering a reamer with the demensions I want though I only plan to build one rifle. Question I have is how to build a dummy round to approximate a .210-.220 freebore? I have a comparator and could build a round with the cartridge base to ogive dimensions based off of the calipers and comparator to approximate the freebore (I understand this isn't very accurate). If I build the round with the Boattail at the neck/shoulder then it will be about a .280 freebore. Can I get some advice?

 

[FEENIX]

I am having a smith build a 300 WSM. I plan to shoot 212 eldx bullets. He has a SAAMI reamer. I've done a lot of research and the consensus is a .200 -.220 freebore. The SAAMI spec is .188. He said I could load a dummy round for him and he can ream based on that OR I hear there are ways of ordering a reamer with the demensions I want though I only plan to build one rifle. Question I have is how to build a dummy round to approximate a .210-.220 freebore? I have a comparator and could build a round with the cartridge base to ogive dimensions based off of the calipers and comparator to approximate the freebore (I understand this isn't very accurate). If I build the round with the Boattail at the neck/shoulder then it will be about a .280 freebore. Can I get some advice?

If you could wait and absorb the additional cost of the reamer, have the reamer specially built for your specification. I have a couple of .300 WSM and have no issues with the SAAMI spec freebore. However, the shortfall for both of mine is insufficient magazine clearance for a longer COAL/CBTO for the heavies. If you are using a SA, Wyatt has a fix if it is compatible with your action. Good luck!

Ed

 

[Boisedoc]

Thank you. I am building off of a Long action - length not a problem. I did read an article (3 inch WSM) where a couple of guys looked at pressure in relation to increasing COAL in a SAAMI chamber and a .250 lead chamber. Long story short, they found increasing the COAL in the SAAMI chamber increased pressure and velocity and increasing COAL did essentially nothing in the long freebore chamber. Made me question just running a SAAMI as all I really want is to hit 2700 plus in a 21" barrel with the 212 and some temp stable powder (H4350 or 4831sc).

https://loaddata.com/articles/PDF/3-Inch%20300%20WSM.pdf

 

[FEENIX]

Thank you. I am building off of a Long action - length not a problem. I did read an article (3 inch WSM) where a couple of guys looked at pressure in relation to increasing COAL in a SAAMI chamber and a .250 lead chamber. Long story short, they found increasing the COAL in the SAAMI chamber increased pressure and velocity and increasing COAL did essentially nothing in the long freebore chamber. Made me question just running a SAAMI as all I really want is to hit 2700 plus in a 21" barrel with the 212 and some temp stable powder (H4350 or 4831sc).

https://loaddata.com/articles/PDF/3-Inch%20300%20WSM.pdf

I think that is possible. I propel the 215 Berger at 2850 FPS out of a 24" with RL-17. I tried both the powders you have and the velocities were not favorable; it does not mean yours cannot be to your advantage - you just have to try it and see how it goes. Yes, most do not consider RL-17 as temp stable, but I have been using it for a few years now without any issues. Good luck!

 

[Rick Richard]

I think that is possible. I propel the 215 Berger at 2850 FPS out of a 24" with RL-17. I tried both the powders you have and the velocities were not favorable; it does not mean yours cannot be to your advantage - you just have to try it and see how it goes. Yes, most do not consider RL-17 as temp stable, but I have been using it for a few years now without any issues. Good luck!

Is yours from a short action?

 

[FEENIX]

Is yours from a short action?

Yes Sir!

 

[JAYgs8163]

I know you mention the 212 ELDX but if you're open to bullet choices also look at the Berger 200.20X . This bullet is the shorter version of the 215 Berger and one of the reasons designed was for the short action magnums. Still has a great bc of 328. I'm in the process of load development on this in my 300WSM. Many reviews I've read of this bullet show good performances on Elk size game animals and off the chart accuracy and great MV. Good luck with your build.

Edit: Saw you're building off a long action. Very nice.

 

[JuddL]

My advise…build the rifle builder a dummy to fit your BDL/DBM setup and let him throat to that. Who cares what the freebore number is, he's going to throat it to your dummy round.

If possible and you have the room, I like getting the pressure ring above the neck shoulder line…realizing that's not always possible with the mag constraints.

Good luck…love my 300wsm.

 

[JAYgs8163]

He's building it off a long action.

 

[Boisedoc]

Thanks everyone. Had a conversation with one of you today that was helpful and gave me something to think about.

 

[cdherman]

I really futzed over a COAL for a 7mm SAUM. Same basic quandry that all the folks with a SA have. BUT Boisedoc is building on a long. I would advocate that he should pick a couple bullets that he thinks are likely candidates for the application he desires, include one of a slightly lighter weight, and load up several dummy rounds with the main body of the bullet a proper .308 into the neck of the cartridge. That can be tricky, but manufacturers give you good diagrams and you can make marks with a caliper to find this.

Send the smith the shortest of these, measured to CBTO. Since you are measuring them all with the same tool, you measurements are internally consistent. You can always seat the longer bullets deeper, but you cannot seat a short bullet shallower, as your run out will start to rise.

Of course, if this a bench queen, you can do whatever you want. But the old wisdom (I know many disagree) is that a good rule of thumb is one bullet diameter into the neck.

The problems encountered with long action (cannot seat bullet to lands, too much jump) are often different that the SA world, where we hit box capacity far too soon. Too much free bore in a LA means you cannot tune the load. TOO LITTLE COAL in any action mean diminished case capacity and less velocity.

 

[Greyfox]

It's a common practice to supply a dummy round when I'm trying to replicate results from an existing rifle that is having its barrel replaced. For a new rifle/barrel, where I'm interested in having a free-bore that is different(longer) from the SAAMI spec, I will get a copy of the standard reamer print. Most smiths should understand your asking for an additional .020-030". If the smith wants a dummy round, it can be made accurately enough using a comparator as long as you are judicious, and account for any added dimension of the comparator to your measurement.
I have done this with good success with my 6.5x284's which I ALWAYS specify. Example:

 

[Boisedoc]

Thank you everyone. This has been an interesting learning experience. There are a lot of factors in play. There is an article in which a ballistician builds a 300 WSM chamber with a .250 throat and a factory .188 SAAMI spec chamber. He loads a 180 ABLR at three different lengths; the last at 3" COAL. He then uses a transducer to measure pressure and he also measured velocity. The long throat chamber sees very little difference in the 3 loads and the SAAMI chamber sees significant. The author concludes from this that the SAAMI chamber is superior. This may be true, but I'm guessing there are more factors at play: (1) gas leak in the longer freebore probably being one. As the PV=nRT equation models what should be an inverse relationship. I did a little math on lengthening the throat using a 212 eldx from .188 to .215 freebore in the 300WSM and get roughly a .3-.45 grain increase in volume. Can figure this out because the bearing surface is still in the neck. I don't know if this is negligible or not. My guess is I am playing around with something that probably doesn't have a ton of real world significance. But it is fun to think about. I do like the idea of throating for the shortest bearing bullet and running with that. The 212 eldx actually has a shorter bearing surface than the 200 I understand. The hammer 181 has a really short bearing surface according to the drawings I've seen. It's almost equivalent to the neck of the 300 WSM so the SAAMI spec is probably ideal for that bullet. (I want to say the hammer is .334 and the neck is .313).