Case mouth clearance 6BR

[MachIV]

Hi Guys

I was playing with the 6BR at the weekend and managed to miss an extremely close rabbit as in 300yd club minus 240 yards! I thought odd - hang on the case was slightly tight to chamber. I didn't think much of it as I had just neck sized some rounds but when I had a look the case mouth was very shiny.

 

Anyway to cut to the chase it seems the chamber length is a touch tight. 1.555 is OK. 1.556 will remove blacking off the case mouth. So it looks like the case mouth is up against the chamber - eek!. The brass is all pretty much standard 1.555 so just OK but not obviously not much margin.

 

My question is how much should I trim them back? I was thinking 10 thou.

 

It is a Border Archer barrel chambered with their short throat reamer if any one has similar experience.

 

Thanks

[Alycidon]

1.56 seems to be the case OAL in my Nostler and Vit manuals.

 

How did the bolt close on the round when you loaded it, was it tightish?. Has the case been shot a few times? , if so the shoulder probably needs pushing back a thou or so. Redding do shoulder dies that do not touch the neck. I use mine about every third reload in PPC brass. If new brass then I dont quite know but I would personally be tempted to take a thou off the neck length. Long necks are benifical for accuracy so dont take more off than you must.

 

Cant see that being the cause of your miss though, put that one down to pilot error I think !!

 

A

[MachIV]

Hi Alycidon

 

Pilot error!! As if!

 

The bolt was tighter than usual to close on the round which I had only neck sized - yes had been fired but only once since using the body die and with a mild load. I didn't think much about it - I thought the same as you shoulder needs pushing back.

 

Basically the miss was high and didn't sound quite right hence checking the fired case. It is the shiny case mouth that caught my eye and made me scratch my head.

 

The manuals do say when 1.560 trim to 1.555. But I blacked the end of a case which was 1.556/7 and chambered it and it took the blacking off the case mouth hence my wondering if the chamber is a touch shorter than usual specification. Guess I need some cerrosafe!

 

Hey I didn't miss the next one after I checked for rounds that chambered normally!!

[PeteL]

Hi Mach,

To check your chamber length, take a look at:

 

www.varmintal.com/arelo.htm

 

He gives a method for doing this very thing. Al's way is easier IMHO, than the Cerrosafe method.

Hope that may be of help

Peter

[baldie]

Ring border and check with them if the barrel was chambered in 6mmbr REM, or 6mmBr NORMA. The case neck length is different. It should also be stamped on the barrel proof mark.

[eldon]

http://www.hevanet.com/temple/MatchPrep.html

 

That should sort you out.

 

If you have access or a friend with a lathe these are easy to make. I have a few and they are simplicity in use.

[Vermincinerator]

It seems to me that Borders short throat reamer also cuts a short chamber OAL

It should not be a 6mmBR Rem spec chamber unless that is "exactly" what you asked for as 6mmBR Norma is now the industry standard (both SAAMI and CIP) plus i think you would meet far more resistance on trying to chamber if it was a Remington spec neck.

Just trim your cases to 1.550 and enjoy your shooting it wont make any difference at all.

 

Ian.

[MachIV]

Hi Guys

Thanks for the info'.

It was pilot error! Must have got all twitchy at the thought of getting in the 60yd club.

 

I looked at Varmint Al and thought hang on lets see if I can find a long bit of brass. So I sifted through the cases that came with the gun and found one that was 1.560.

 

It was very tight to chamber so I body sized it and it chambered fine - so looks like the shiny case mouth theory is not a good indicator of chamber length. Learnt something new which is all part of the fun.

[onehole]

Your round probably went in low and together with being a bit cocky and not taking care even with a close one you really went in low?What I am saying is your bullet is still climbing to meet sight line with your chosen zero? .Been there done that and missed a bigger target than that at close range,not saying what!!!????cheers onehole.