6CM testing

[JERRYHM]

Results of 100 yard testing of the 6CM.
1000 yard testing will start on 8-14-06

6CM LOADING NOTES FOR REFERENCE ONLY 01 August 2006

Reamer used was the 6CM LR a version of a 243 Winchester improved.
Action: BAT single shot model B
Barrel: Hart 6mm 7 twist 1.250 X 4” standard taper to 0.875” 30” long with 6 Flutes.
Bench rest mode with a Night Force BR 12 – 42X scope. The rifle weight is 17.25 #.
Winchester 243 Brass was prepped and sorted into lots by weights. Neck diameter new and resized is 0.269”, loaded is 0.271” and fired is 0.273”.
Sierra 115-grain bullets touch the lands with an OAL of 2.915”. Berger 115-grain bullets touch the lands with an OAL of 2.972”.

Head Space measurements --- The rifle chamber HS with the 6CM gauge is under +0.002”. I can close the bolt with a +0.006 243 gauge. The new 243 cases have an HS of about –0.002”, meaning the HS is about 0.008” with new win brass. This is causing the failures to fire. Reloaded the ladder/chronograph test using a false shoulder,necked up the brass to 6.5mm and necked back to 6mm until the bolt just closes). This BAT action will not fire with an HS of more than 0.004”.

11 August: New test with fire FORMED cases.
Sierra 115 loaded 0.020” short of the lands. 75 degrees, no wind, elevation of 200’

POWDER – Weight – Primer – MV – ES – SD
H1000 48.0 CCIBR 3147 20 07
H1000 48.0 CCIBR 3153 26 10
H1000 48.5 CCIBR 3194 22 08
H1000 48.5 CCIBR 3188 13 05
H1000 48.5 WLR 3208 12 05
H1000 49.0 CCIBR 3239 19 07 *** some pressure signs starting to show
H4831 43.0 CCIBR 3033 33 13
H4831 43.5 CCIBR 3065 23 09
H4831 44.0 CCIBR 3103 27 11

I found a problem with the barrel and edited post to remove the five shot group sizes. The MV's are OK, it is just not a good barrel.

 

[rcw3]

Jerry

I'll tell you flat out I have had very bad luck with 1"7" twist barrels in 6mm's, and I know two other people with the same experience,both with Hart barrels). I currently have a 1:7" twist Hart and a Krieger and both are problematic. Personally I think a 1:7" twist is too fast. The 1:7.5" twist barrels I have seem to shoot fine. I just ordered a 1:7.8" twist 6mm barrel for a project. Frankly with the 115's I think a little less than 1:8" twist is about ideal. The 115's are, in my opinion, approaching the biggest bullet the 6mm bore size can handle, and for everything to work well everything needs to be just right,barrel twist included in this equation). I am sure there are those that will take issue with me on this, but personally I will never order another 1:7" twist 6mm again,too close to the edge - and there is a limit to an effective bullet size for the bore, an effective twist as well, etc.). People seem to think there is not a limit and a few more grains of bullet weight and a little extra twist won't hurt, but I have done a lot of testing over the years and "pushed the envelope" on some of my projects, and I can say for sure,in my mind) there is a limit, and pushing 115 6mm's out of a 1:7" twist barrel is getting there. Maybe you can tweak it and things will ultimately work out, but what you may also wind up with is something that is always hanging on the edge and not a consistent day in and day out shooter,i.e. it shoots like a house on fire one day and not so good the next - and who needs that).

We must keep in mind that the DTAC 115's came out and many people rushed to purchase and shoot them, but overall there was very little testing relating to the bullet and very little information available as to what's ideal and/or appropriate. Tubb also never said he shot a 1:7" twist with the bullet either,i.e. he indicates he shot 1:8" and 1:7.5" twists).

Robert Whitley

 

[lee_eastridge]

Jerry.


My good friend bought a 223 barrel in a 6.5 twist and it will
key hole bullets at 100 yds, 77 g sierras.
the other barrel was a 1 :8 twist and it was a tac driver.
maybe over rotation. ?

hope this helps.

Also, i shoot a t2k in 6xc with a 1 7.5 twist barrel
I went from sharpshooter to a high master in 1 weekend
the rifle shoots well every time period.
combo molly 115 DTAC ,win brass,Russian primer,h 4350
2.750 or jump them it does not matter. since that weekend my long range average has been 98 to 99%

 

[joesr]

Jerry and I talked before he posted these results. I told him to go ahead and post the results as I don't want to hide anything about this cartridge.

However.....

Based on the fact that his 210M primer lot was bad and that he is thinking the barrel may not be a good barrel, I'm not sure what the testing proves.

With the 6CM I have only used Kreiger barrels to date and to date I have not had a bad one. I will be shooting a Satern Barrel in 5 groove with canted lans and a Bartlein barrel in the near future. However, the Kreigers have proved to be great.

Furthermore, there has never been an issue, with any of my customers, in getting fire forming rounds to fire.

I have also tried a Hart 1:8 barrel and it would not shoot the 115's. I think a 1:7 should, but I don't know.

What I do know is Jerry is working outside of the parameters I have set for the 6CM.

These results are not representative of the group of barrel I have chambered.

Joe Hendricks

 

[rcw3]

Joe

I had no problem shooting the 115's out of two different 1:8" twist barrels,1 Hart and 1 Kostyshyn). I did purchase a Broughton 1:8" twist barrel which was actually more than the 1:8" twist it was labeled,i.e. about 1:8.3" twist or so) and it would not stabilize the 115's sufficiently. I'll go back to what I indicated before, that the 115 is a bullet that is about as big a bullet as a 6mm bore can handle, and for everything to work well everything has to be just right and in proper proportion and balance,barrel twist and everything else included). For barrel twist with the 115's, I believe somewhere in the 1:7.5" to 1:7.8"twist is about ideal. A 1:8" twist will do it if the barrel is a true 1:8" twist, but that can be a problem for some button rifled barrels that don't come out with the exact twist,i.e. Hart, Broughton, etc.). What I am discussing is not,in my opinion) a phenomenon that has anything to do with the 6CM, but with shooting the 115's from 6mm bores.

By the way Joe, nice shooting at Perry.

I would just caution shooters that 1:7" twist 6mm barrels may be problematic. Follow the age old benchrest shooters advice of using the slowest twist barrel that will adequately stabilize the biggest bullet you intend to shoot,i.e. and no more).

Robert Whitley

 

[mod70]

What is the water capacity of the 6CM,and the 6XC? Thanks

 

[JERRYHM]

Joe sent me one of his Krieger barrels 1:7.5 twist and it has been installed. The results are very good. The Hart barrel was inspected with a bore scope and a couple of problems were observed. One land about 20 inches down from the chamber has a 2-3 inch burn section on top of the land, some kind of defect or hard spot). I have spent a couple of hours trying to clean this barrel without any success. I have never seen a barrel look so bad with so few rounds,220) through it. I have almost that many rounds through the Krieger and it still looks like new.

6CM LOADING NOTES FOR REFERENCE ONLY

Reamer used was the 6CM LR a version of a 243 Winchester improved.
Action: BAT single shot model B
Winchester 243 Brass was prepped and sorted into lots by weights. Neck diameter new and resized is 0.269”, loaded is 0.271” and fired is 0.273”. Sierra 115-grain bullets touch the lands with an OAL of 2.918”.

NOV 10, 2006
New barrel installed, Krieger 1:7.5 length is 28”
Load test at 100 yards on 11-10-2006 at Sacramento temp was 40 degrees.

Powder = H1000, Primers = CCI-BR2, Case = Winchester, OAL = 2.900” jumping 0.020”, Sierra 115 grain bullets without the black garbage on them.
Chronograph numbers are questionable with the dim light.

47.0 grains MV=2956 Group = 0.426” five shots
47.5 grains MV=3010 Group = 0.331” five shots
48.0 grains MV=3066 Group = 0.375” five shots
48.5 grains MV=3089 Group = 0.397” TEN shots ---- Round count thru barrel = 65
No pressure signs. On the next test round will try 49 and 49.5 gr. and changes to the OAL.

First Match ----- three 600-yard prone matches, iron sights 11-11-2006
Conditions at Sacramento 40 degrees, light rain, dim light, heavy wet wind flags that could not show the changes.
Match Winner with a 598-30X
First 600 using 47.5 gr. = 200-10X
Second 600 using 48.0 gr. = 198-09X
Third 600 using 48.5 gr. = 200-11X
The 48.5-grain load did shoot the best; all three loads shot very good elevation.
Round count thru barrel = 131

The Sunday 1000 yard matches. 11-12-2006

Any-any match I was one X behind first place with a 199-10x

There were NO elevation problems at 1000 yards.
Round count thru barrel=182

 

[sparker]

Hi Jerry,

It looks like you have probably reached the end pressure wise. Your velocity increase was steady at~50 fps/half grain. When you went to 48.5 grains, your velocity increase was cut in half. When this happens to me, I typically have my load. Pressures seem to go up remarkably from there.

Scott Parker
Bakersfield CA

 

[JERRYHM]

Scott,
I agree the 48.5 grain load might be near the max. One concern is the chronograph, the light was so dim, I just do not trust the numbers there were many proof channel errors.

I was shooting next to a 6.5-284 at 600 yards and on a wind change we both lost points the 6CM,9) was about 4 inches inside of his,8).

 

[Dgd6mm]

Info........

 

[cmillard]

robert, regarding twist rate--i have said the same before. i have talked folks out of getting 1-6.5" twist for their .223 rifles. i have told them that it is too much twist. they fire back saying that the bullet box recommends that twist. so i tell them that i have successfully shot 90 berger vlds out of an 18" barrel with a 7" twist.

 

[Shootinstick]

I have had 3 28" Pacnor 6.5 twist 223 Remington barrels from White Oak on my AR15 space guns. I shoot 77grain Sierra SMKs at 200 and 300 yards and Berger 90's at 600 yards. All have shot well with the 77's including setting the current ND state rapid fire record (200-14X) at 300 yards a couple years ago. Compared to my service rifle with a 7.7 twist Krieger the Pacnor 6.5 twist barrels shoot much tighter groups with 77grain Sierras. YMMV

 

[Dgd6mm]

For those that are wanting info on the 6CM, this is some good stuff.