6BR freebore question once more

[bushpilotmexico]

First I apologize because I know this topic has been discussed many times but this is my first experience with 6BR. I have a Savage 12FV .308 and I ordered a prefit 6BR Super Match barrel from PacNor. They chambered it for the 6BR Lapua case. Not knowing anything about freebore I never asked and when the barrel arrived they had noted that freebore was .115-.120.

When I searched for freebore postings last night and reading the threads the general feeling was that any freebore above .115 might work with the 105 VLD bullets but may not have sufficient neck to grip the 70 grainers, etc. It seemed like .060 was an ideal compromise.

I have an OAL gauge and a Sinclair comparator as well as a Sinclair 6mm chamber plug to determine chamber length.

When I first received the barrel I was experimenting with all kinds of loads and bullets including 70 gr MK's, 69 gr Bergers, 87 gr V-max and so on and to be honest all of them had lots of neck to grip the bullet.

This morning I went out to the shop to try and figure out this freebore issue. I went back to my OAL gauge set up and here is what I came up with using standard cases trimmed to 1.555 which I calculated gave me a total neck length of .312 plus or minus a couple of thousand.

1. Berger 105 VLD ....they gave me an OAL of 2.620 to the lands and seated .284 into the neck which I calculated was .028 above the neck/shoulder junction. Would this clear a possible donut situation?

2. Hornady 87 V-max ....they gave me an OAL of 2.620 to the lands and seated .334 into the neck or .022 past the neck/shoulder junction,definitely into the donut area).

3. Sierra 70 gr MK ....they gave me an OAL of 2.634 to the lands and seated .215 into the neck or .097 above the neck/shoulder junction.

I know last night the general feeling was that the case neck should grip an amount of the bullet equal to it's diameter.

What I decided to do next was measure the actual length of my chamber using the 6mm chamber plug and a brand new Lapua case which measured 1.555. Needless to say I couldn't chamber the round as the plug was bottoming out on the case neck. The Sinclair 6mm chamber plug has a shoulder which measures .100

What I did next was to radically shorten the Lapua case, insert the plug and re-chamber the round. This time the bolt closed. When I removed the round the OAL was 1.578. If my math is correct when you subtract a case length of 1.555 the difference is .023 which if I am not mistaken, and I might be, should represent the actual freebore because the 6mm plug should stop when it touches the lands or entrance to the barrel.

I know this is a long posting and I apologize but am I correct in my calculations then in fact I don't have the .115 - .120 freebore that PacNor says and actually have a freebore of .023?

 

[bushpilotmexico]

With your 87 V-max if you have .360 into the nexck and are still above the neck/shoulder junction then how long is your actual case neck? Mine is only .312 the V-max seats .334 into the neck which is .022 past the junction.

If I turn a plug on my lathe 6mm or .243 and insert it into a case and chamber it then take it's OAL and subtract the actual length of my chamber,1.578) as determined by the Sinclair chamber plug the difference should be then the actual freebore?

Does this sound reasonable?

 

[bushpilotmexico]

I also forgot to mention that my 6BR barrel has a .272 no turn neck.

The chamber plug that I used was modified on my lathe for my 6PPC with a .262 neck so the actual diameter of the head of the plug is .2615 in order that I could chamber it.

If the neck of the 6BR is reamed .272 wouldn't the .2615 diameter plug just slip on thru til it conatcts the throat of the barrel?

I'm just new at this so please don't mind my newbie questions.

Thx

 

[bushpilotmexico]

I went ahead and turned a plug on my lathe the same diameter as the Sinclair chamber plug .2415. I chambered it in the rifle and it's overall length was 1.598....the length of my chamber using the chamber plug measured 1.578 so the difference is .020.

I can see visually that the diameter of the plug I turned,and it has to be to seat in the case neck) is smaller than the neck diameter. With my .272 no turn chamber neck wouldn't the plug have to be hitting the throat of the barrel after it passes thru the freebore space?

Other than casting the chamber how else can freebore space be measured?

 

[bushpilotmexico]

"If you did that it should have fallen into the case... was it 0.2615" instead?"

The actual diameter of the Sinclair chamber plug is .242 and that is the diameter that I turned my plug.

If you use the micrometer on a brand new Lapua case the inside diameter is .240 and the neck thickness is .014.

Naturally a .243 bullet measures .243. Double .014 = .028 and add this to the bullet diameter = .271 ....I have a .272 no turn neck.

The plug I made at .242 slid into the unfired case with a moderate amount of resistance.

I was away for the afternoon and have to study your jpg of the 6BR reamer...thank you very much.

If you had a freebore of .110 and wanted your gunsmith to deepen it to .125,hypothetical situation)then how would he measure the exsisting freebore and what size reamer would he use?

 

[MarkS]

Sounds like you are ok on your bullets. I have a 10 tw 6BR throated for a long 105gr FB bullet. I have small gr bullets, just 0.20" into the case, and shoot great. You did not say what bullet you wanted to shoot.

Mark Schronce