may have figured something out- why winchester hmr ammo doesn't work in my cz

[cartwheel]

So I had some friends out to the range and the hmr is a go-to for me as new shooters all like it. Had some winchester kicking around- I don't shoot it because it doesn't shoot well for me, but for newbies it is fine, no point burning the good stuff.

I kept some shot brass after I found a neck split. I also had some hornady vmax which got shot and I kept a few cases as well.

Measuring the necks on fired cases I found they were 0.193" which I guess is the diameter of the neck in the chamber.

So then I measured some unfired rounds... Hornady was 0.1915". Winchester was... 0.193"! So virtually the same size as the chamber. I think that Winchester brass must be slightly thicker and the cz chamber is tight at the neck, so I'm thinking this is why the winchester doesn't shoot for me- just not enough clearance.

Anyone want to trade a few boxes of winchester for CCI/hornady?

 

[kjohn]

Hmm. I have some Winchester stuff and actually shot some off this afternoon. Never thought to keep any brass or ask the blown apart gophers if it was any good.

 

[LUTNIT]

There is spring-back with brass that you have to take into account. If there wasn't, cases wouldn't extract without a hammer and rod down the muzzle.
I googled some drawings and most show .197" or .1985" for ideal chamber dimensions in the neck. That's US specs, not European.

Without a chamber cast I would be hesitant to say the neck of the chamber is .193" based on a fired case. It's probably close, I can't see properly annealed .17 cal case springing-back .004" so it is probably somewhat close to .193".
First thing I'd do is get a micrometer instead of a caliper and get some proper measurements to 4-decimal places. Cheap calipers have an error sometimes as large as +/- .002" so measurements this fine are best done with a micrometer.

There was one guy here on CGN who had issues with CCI 22lr ammo in a CZ rifle. When he contacted CCI about the problem they said to try the ammo in a SAAMI spec chamber instead of a CIP one as that is what they manufacture their ammo for. I don't know what kind of tolerances/differences there are so can't say if that was a BS excuse for CCI or if there really are differences with CIP spec firearms.

 

[cartwheel]

I'm familiar with spring-back, but it is a few tenths, not thou. I broke out my trusty 0-25mm mic and had at the fired cases:

4.89mm
4.87
4.88
4.86
4.86
4.87
4.85
4.88
4.92
4.92
4.91

Note- the last 3 were Winchester cases. Now for some loaded rounds:

Hornady 17gr Vmax: 4.86mm- repeated over multiple rounds
Winchester 17gr Vmax: 4.90mm- repeated over multiple rounds

If I were to guess without a chamber cast, I'd say the neck was around 4.93mm or .194". It would seem that the hornady rounds expand the neck a few thou while the Winchester have probably less than 2 thou of clearance.

Anyway- found it all interesting, and I think it does explain why my rifle doesn't shoot the winchester well, just not enough clearance. It also probably shows that the chamber in my rifle is probably no to spec- but I haven't found a drawing for the hmr yet.