Reading various people talking about converting Mini-Mauser actions like the CZ-527, and in the process of doing it myself, I wanted to add a little on safety.

When you buy a CZ-527 action with a 7.62x39 bolt face from CZ (not the rifle, just the action), the action is marked 3,550 bar. This pressure converted into pounds per square inch is 51,488.4 psi. This is the same pressure that the 7.62x39 case has. Because I at first questioned the stamp, and I wanted to run a Grendel at higher than 51k psi, I ran some numbers and here is what I found:

As we know the CZ-527 actions are chambered for various rounds, there are three main case head diameters of those rounds from 22 Hornet family size up to 7.62x39 family size. The max pressure for a factory round you can buy a CZ-527 in is 62,366 psi through its .223 Rem chambering. As many of us know, the pressure in a case being fired is applied outwards against the surface area available. It is essentially a externally supported pressure vessel at this point. The .223 Rem has a case rim diameter of .378", and the area of that circle equals 0.1122" inches squared. The pressure of 62,366 psi on that area therefore equals 6,998.76 psi if my math is correct. Again as many of us know, this value is what the term "bolt thrust" refers to.

The 6.5 Grendel case, and its parent case the 7.62x39 have a case rim diameter of .445". The area of that circle equals 0.1555 inches squared. The 7.62x39 max pressure is 51,488 psi which results in 8,007.85 pounds of bolt thrust, and somewhat more than generated by a maximum pressure .223 Rem round. Running a 6.5 Grendel up to say 60k psi of pressure like one might run a very hot .223 Rem load generates 9,331.71 pounds of bolt thrust. That is 133% of the pressure from that maximum load in the .223 Rem case.

In an larger action, perhaps like a Remington 700, it would never be an issue. That action has a large lug to action engagement area that will handle a case rim diameter of .532 -.534" at 63k psi. However in a CZ-527, with its much smaller lugs, it is a different story. To my thinking, this is likely why CZ has stamped the 7.62x39 CZ-527 models they sell as actions only with the 3,550 bar pmax rating. My other CZ-527 rifles (and I have quite a few) including the 7.62x39 Carbine model don't have this stamp.

Without getting into all the details of multiple cycle, fatigue related yield strength reductions, I'll just say that running higher pressure than designed not failing in the short term (i.e. a couple hundred rounds fired), doesn't mean that running higher pressure than designed is safe in the longer term (i.e. a couple thousand fired).

Also, if there is a desire to run the Grendel at 60k in a benchrest action, or a Rem 700 action with a small bolt face opened up to fit, here is a little on the 6.5 Grendel case from a post by Bill Alexander. He designed the 6.5 Grendel (with some help from others, and I am not wading into that). The post is from the 6.5Grendel site:

quote:

Grendel has the MAOP set at 50,000 psi. This was established as the ideal working load for the rifle to allow for a durability cycle that matches the existing military needs for a fleet size sample. It may be a little conservative. The maximum pressure for the round at STP is 51,800 psi. This will account for production variations, storage variations for a batch and to a certain extent a temperature/instrument wander.

The case is developed with a 58,000 psi web. Depending upon manufacturer the offset for the case varies between 5,600 psi and 9000 psi which is an indicator of the ability of the case to operate in a automatic mechanism, which opens with some level of residual pressure.

The rifle itself is rated to the same as the case at 50,000 psi for operation but has to also be able to fire proof loads without sustaining damage. In this instance 67,340 psi. The difference between the MOAP and the proof loading constitutes the safety factor and allows for the durability.

I am not saying I am right about this. Among other things, I don't have exact lug area measurements for the CZ-527, and of course my math could be wrong. What I would like do (and hopefully have done) is raise some awareness. When I saw the stamp, it got me thinking about why it was there; this post is what resulted.

Webfeet, that sounds like very good advice and you gave an excellent discussion.

Regardless of the action used, the information provided by Bill Alexander tells us that shooters who load this round to 60,000 psi are loading it way above the 51,800 psi at STP maximum design pressure. In fact, prudent loaders should not be up striving for loads over the design MAOP of 50,000 psi.

Yet we know that some people will load to 60,000+ psi anyway. The majority of them will be able to get away with it because they don't shoot very many rounds and/or their rifle and brass are made with sufficient margins of safety to protect them from their folly. Have no doubt, though, that if their brass fails they will blame the brass manufacturer and if their rifle blows an extractor, or worse, they will blame the rifle manufacturer.

Seems like everyone wants a hotrod. Wait I blew a number of primers in my youth.

I would load the Grendel to the Pressure it was designed. If what you can get at 51,000 won't get it done stressing everything wont either. Need more that the 51,000 will give you go with a larger case.

Spot on webfeet.