Warning

[tim simbari]

I'm not sure how many are or are not aware of this but it has serious safety issues. This pertains to Cooper 20TAC chambers and Lapua brass. A buddy just got a new Cooper 20TAC and started loading with lots of pressure at fairly mild loads. A conversation with Rob yielded the fact that the reamer used is with a neck dia. of something in the order of .2305" or so. This will NOT allow Lapua brass without turning the necks which really isn't needed otherwise with the Lapua. My Dakota is .233 or so which gives plenty of clearance. Just an FYI.
P.S. I just talked to Todd Kindler on this subject. Cooper has apparantly used an out of spec reamer for TAC 20 guns. The neck runs 2 thousanths undersize.

[Nelson]

I have a Cooper in 20 Tac comming and should be here anyday, I also have bought some of the Dakota/Lapua 20 Tac brass, so will I have to neck turn? or is it just when necking down the 223 Lapua brass?

[tim simbari]

I've only heard this second hand, but it's sounds like Cooper will re-chamber if you wish. If not, turning the necks eliminates the issue.

[ed3645]

I have a Cooper Tac 20 and just went to the range yesterday to break in the rifle and develop a load. I did not have any problems with pressure and the rounds loaded and extracted without any problem. I measured several rounds and came up with the following measurements.

Dakota Brass:
New the neck measures .228
Fired in Cooper it measures .2335 to .2340

Neck wall thickness is a consistent .013

Loaded rounds measure .230

The hottest round I shot was 26.6 gr of H335 (Hornady book shows max is 26.7). I shot 15 of these, I just looked at all of them and don't notice any signs of over pressure.

[Alex]

the FIRED IMI brass measures

Varminter --- 0.234
Classsic --- 0.233

The loaded

Dakota brass ------ 0.230
IMI parent brass --- 0.229
Winchester pt. br. - 0.0228

The easiest thing to do would be to get some Winchester 223 brass and form a few cases and measure the FIRED diameter of the necks. This would tell you if the Lapua would be safe.

Winchester brass is usually very consistant. Sometimes not quite as tough as the IMI, which is no longer available. The "tight" necked reamer used by Cooper and the Winchester brass would be ideal and not a problem. Does mean that guys buying the Dakota stuff wasted their money if they have a "small necked" Cooper 20T, unless they wish to neck turn.

I didn't bother to measure the wall thickness, as it is the loaded thickness that is the most important. When I have a few minutes, I'll measure the neck concentricity on some of my IMI, Dakota and Winchester 20 Tac brass and report back.

Alex

[foxhunter]

may not mean a thing but my 221 fb brass formed from lapua weighs 10 grains more tham remington brass, they were made from 223 lapua brass.

[tim simbari]

I would guess from your reports, they have used more than one reamer. They are absolutely aware they have guns out there with necks that are not much biggger than .230 as opposed to the .233 they should be. They should deffinately be checked.

[foxhunter]

catshooter, the cases were not turned as the rifle it was being shot in has a sloppy neck. the fired case measured .255, resized measured .249 and loaded is .2505.
the case wall is .0145 as compared to the remingtons i measured at .012.