6 Creedmoor chamber oversized?

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cc737

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Okanagan, BC
6 Creedmoor load 40.3 gr H4350 , cci 450m, Berger 109 LRHT brass is Lapua small rifle primer
I have no heavy bolt lift or extraction problems
I was told the reamer has a .472 base measurement .200 from from end. head space is between .003 and .0045
on first firing base measured .300 from end of brass were all over .473 and up to .475
on my second firing base measurred .300 from end of brass all brass is .474 and upto .47505
I have taken a new piece of brass rim measuement of .469 and inserted it backwards into the chamber for a measurement of .895 past the go gauge end

My feeling the chamber is oversized and may not be safe.
 
Seat a projectile on a new piece of brass to a hard jam, resulting & witnessed w/ rifling impressions at the ogive.

Reduce your powder charge by 1 grain.
Shoot & measure the case web at the .200"datum line.

(impossible to measure to the 5th decimal mechanically)
 
cc737 said:
6 Creedmoor load 40.3 gr H4350 , cci 450m, Berger 109 LRHT brass is Lapua small rifle primer
I have no heavy bolt lift or extraction problems
I was told the reamer has a .472 base measurement .200 from from end. head space is between .003 and .0045
on first firing base measured .300 from end of brass were all over .473 and up to .475
on my second firing base measurred .300 from end of brass all brass is .474 and upto .47505
I have taken a new piece of brass rim measuement of .469 and inserted it backwards into the chamber for a measurement of .895 past the go gauge end

My feeling the chamber is oversized and may not be safe.
Click to expand...

I read this twice and can only come up with, WTF are you talking about?

You stuck a piece of brass BACKWARDS into the chamber? W.H.Y ??????

IMO you are WILDLY overthinking this. Just shoot the thing.
 
What speed is it running and what was the length of the barrel again ??
My barrel on the same reamer as what yours was cut run 3100 on Federal gold medal match with a berger 105 gr on a warm day and brass measures the same .474 to .475 and is 28 inches. Your charge weight and a magnum primer will be in the 41.3 range as a magnum carrys approx 1.0 grain in powder charge weight more then a std primer.
 
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Most of the information in your first post doesn't seem to really provide any details... and sticking a case in the chamber backwards truthfully doesn't make any sense. That is not, and never will be an accurate way of measuring anything, let alone comparing whatever random measurement you found to a headspace gauge... which is a precision ground measuring instrument, not a malleable piece of brass.

You said "head space is between .003 and .0045"... is that .003" to .004" above/below, minimum, maximum, or somewhere in between? SAAMI states the 6mm Creedmoor Headspace Min is 1.5410", and Max is 1.5510". That's a difference of .01".

Your measurements taken .300 from the base of the case don't make any sense either, that purely seems to be an arbitrary distance. The SAAMI reamer drawing shows a base diameter of .4727 (+.002" tolerance). .200" from the base shows a diameter of .4714 (again, + .002" tolerance) Furthermore, the base diameter has nothing to do with headspace... headspace is measured from the cartridge seat to a theoretical datum established somewhere along the cone of the shoulder - in the case of the 6mm Creedmoor, that's a diameter of .400" at a distance anywhere between chamber MIN and chamber MAX.

Nothing you've posted would lead to assuming the chamber is oversized.. it sounds like your barrel and chamber are perfectly fine, and you're possibly confused, or measuring things that don't need to be measured.

Hopefully the above helps you out!
 
#1
I started my loads at 39.0 gr with a 3gr increase to 41.6gr h4350 had no primer or sticy bolt
the measurement at the datum line .200 is .477
I am measuring using a micrometer holding stand, Mitutoyo #293-811 witch will measure to .00005 also have Mitutoyo 167-912 standard set 1 inch to 5 inch, set to zero crank out and measure at 1 inch mine is right on
#2
I do not have a gage pin set and was just using the end of the brass measurement of .469 as to how far it would go in
#3
I do not know my speed, barrel is 1.250straight for 8 inches tapered to .935 overall lenght of 28 inches
#4
My concern is having fired brass at .474 or more and resizing to .4695 over and over
#5
my head space is between 1.543 and 1.5445
 
Just for fun, I pulled out some fired old brass from various rifles over the years and measured the case diameters near the base. Now I only have a pair of calipers but this have served me well for many years. All brass use large rifle primers... and all brass has been fired multiple times.

6.5 Creedmoor was all Hrn brass using H4350 and 140gr type bullets. At least 3 barrels... maybe 4?

22 Creedmoor was Starline, Hrn, PPU, and Peterson. These were not run at super high pressures but have been fired in Bolt, Semi and Straight pull actions. Fired from at least 7 barrels. And I have run some brass in multiple barrels.

This is fired brass and not cleaned nor resized... just how they came out of the rifles and then stored. The average diameter for most of the brass whether 6.5 or 22 CM was 0.471 and 472... only 1 batch was 0.473 and was in a bag marked "Strained brass". I did have one batch that was 0.470"

YMMV

Jerry
 
I am using Forster dies for both 6 and 6.5 creedmoor
full lenght sizing (this one is shared between them the 6 and 6.5 also share the same go/nogo gauges)
bushing bump neck sizing
ultra micrometer seater die
my 6.5 3 times fired lapua small rifle brass measures .469 at datum line
 
cc737 said:
I am using Forster dies for both 6 and 6.5 creedmoor
full lenght sizing (this one is shared between them the 6 and 6.5 also share the same go/nogo gauges)
bushing bump neck sizing
ultra micrometer seater die
my 6.5 3 times fired lapua small rifle brass measures .469 at datum line
Click to expand...

How fast is that one going ??
 
by using JBM ballistics with BC and recored drop at various distances
140 gr Berger hybrid, 41.4 gr h4350 27.75 inch, 1in 8 twist barrel, speed calculates to be 2880

the only time that I have had problems with shooting was with a 260 rem improved at the lowest load I was blowing primers and my cases were expanding.
I had the threads cut off (.981 of an inch) and rechambered to a straight 260 with no problems.
you could take a piece of brass and it would fall through the cut off piece from the barrel
so the same relults will happen with my current 6Cr if I cut off .875 from the barrel
So that is the reason for inserting a piece of new brass backwards into your chamber
 
Means zero
Have to chronograph for your barrel and the fact the brass dropped through your old 260AI portion of cut off barrel means noting in relation as to the issues you had with it.
 
dan belisle said:
Get a chamber cast and measure it. Compare to specifications. Sorry, the rest of your backwards brass method doesnt really tell you anything. - dan
Click to expand...

Chamber cast is definitely the best way but the backwards case will give you some useful info. Like measuring the throat length using pin gauges... nothing precise but if you are expecting X and you get 2X, then something to follow up on.

OP knows the diameter of the rim of his case... we have SAAMI drawings which will give you a good idea of where that diameter should be in the chamber. If the inserted depth is much further then expected, odds are the chamber is larger then expected.

simple

Jerry
 
When the OP was told the reamer print was .472 that is what the print states but also that reamer can be .002 over that print and still be with in spec of manufacture as it is common to build .001 to .002 over sammi spec print to maintain the life of the reamer for nominal wear and use. If everything was build to spec and the smith used it any amount the reamer will wear to below spec and chambers would be undersized and margins lost at key areas on the reamers. Chamber is 100% safe in every respect.
 
IMG_6057.jpg
I am thinking that Jerry is getting what my point of view is by inserting apiece of brass backwards
The image above is the cutoff from a 260 rem and is .960 of an inch long
the brass that is inserted in the correct direction in a lapua 284 Win, 1.10 inch most are saying there is no problem with the chamber
I happen to disagree, check the SAAMI size of a 284 win
how many of you are able to insert a 284 case this far into a 260, 308, 6cm, 6.5cr chamber
6cr chamber max at datum line .4714 plus .002 for .4734
 

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