Well today I received some 10mm Magnum Brass from Starline.  I trimmed one to the maximum headspace dimension from the SAAMI specs which is 1.004".  The second was gradually trimmed until it would chamber and fire in the Mech Tech.  That dimension turned out to be 1.018".  I believe this indicates that the headspace for my particular CCU is excessive. 

I tried both cases in my stock Glock 20.  The case at max SAAMI specs did chamber and fire, while the longer case would not.  What other conclusions am I to draw?

Quote from: The_Shadow on November 08 2013 10:07:57 AM MST
I doubt I'd go to 700X, but Power Pistol has some great applications.

I have to wonder why you are seeing the separation issues, when other have run the Underwood and Double Tap ammo thru theirs.  Can you contact MagTech to explain your situation or ask if there is a way to delay the ejection cycle?  Could the recoil setup be for a 40S&W rather than a 10mm?  Wrong spring? or detent for lock up?  I am not that familiar with the design and how they control the timing of the ejection.

Best of luck getting things worked out...

I have been in contact with Mech Tech a few times. They are stumped as nobody else has reported problems. I thought about the spring, but I understand it has little affect. Here is a link to explain:

http://www.orions-hammer.com/blowback/

We will see once I check the headspace.

Quote from: The_Shadow on November 07 2013 04:24:48 PM MST
Yes 1.0040" is a MAX depth and 0.9920" is shown as Minimum depth of the final Chamber measured from the breech face.
http://www.saami.org/PubResources/CC_Drawings/Pistol/10mm%20Automatic.pdf 
Difference is 0.012" or 12 thousands.
Brass being very short would likely have a light primer strike with a long chamber and a short brass.

If the Bolt face to chamber is correct in headspace, you could still lack some support around the casings as seated in the web area.  Therefore with higher impulse loading you may see expansion in that section of you brass.  Case separation can come from over expansion, weakened casing or both.  Another thing that I have seen in semiauto rifles, is when case expansion is at maximum and being pressed by pressures tight against chamber walls, ejection starts to occur and the ejection rims or even case heads can tear off.  Slow burning powders are culprit as pressure remain high during the cycle.

Have you checked to see how much case support is actually there?  With the upper removed and a dummy cartridge loaded in the chamber, bolt face against the dummy cartridge you should see if there is any lack of case support.

Just trying to describe what could happen, not so much that it will happen as a method of what to look for.

I agree with you. I have not measured the chamber depth to bolt face yet. I contacted Starline and they are sending me a couple of 10mm Magnum cases for me to trim and use as headspace gauges.

Case seperation has occurred only with slower powders such as AA#9, Longshot, and Blue Dot. These loads were 95% of book max. The cases which did not seperate at this loading were significantly stretched after firing, approx 0.998". Some had primers protruding from the primer pockets. What's strange is there is not case bludge or smiles as I would expect. There is lack of support in the chamber, about equivalent to a factory glock barrel.

I may just stick with faster burning powders like 700x and Power Pistol if i can make the longer burning powders work.

Quote from: The_Shadow on September 11 2013 07:15:38 PM MDT
Well the Mech Tech is what it is...semiauto makeshift carbine.  Tolerances may not be precise for the high impulse ammo use.
Pushing 200 gr bullets much over 1200 fps from the stock pistols have even presented issues in several makes of pistols.

This is where people have added aftermarket barrels, recoil systems in the effort to attempt to control the high impulses generated.  If you barrel chamber is loose, or lacking support, or the head spacing, is not correct these things will further complicate things.  You will have to take a good look at each starting with the head spacing in relation to the breech face and cartridge fully into the chamber.  You will want to work with brass cases that are measured around 0.9900" and be within about even or less than 0.0004".
0.9900
-0.9870
0.0030

I am not familiar with the locking system that is being used by the Mech Tech and how it relates to the head spacing and lock-up.
They trying to determine if your slide timing is accelerated beyond the recoil systems' capability given the impulse of the ammo in use.   

How do you get these dimensions? The SAAMI specs for maximum headspace is 1.004". The minimum case length is 0.982". A difference of 0.022". I am trying to find some longer brass (ie, longer than 1.004") to use as a no-go gauge to see if my chamber exceeds this, therefore, causing my shorter cases to expand and ultimately seperate.

I had to send my mech tech back for repairs. The upper site rail came loose. One of the all thread studs weld broke. I was going to have a local fabricator grind off the current studs and fully tig weld some larger diameter ones, but was worried the heat would warp the inside of the housing causing the bolt not to move freely and likely void my warranty.

The people over at mech tech were great though. Reimbursed my shipping costs and are currently fixing the studs. Hope they fully weld them this time and not tack in place.

I have tried all of the loads mentioned above. Started noticing pressure signs with longshot around 9.2 grains so work up slowly. 150 yards with that load has not been a problem for me.

These videos are how I found out about mech tech. I was hooked the first time I watched it.

Mech tech does have a hefty markup on their accessories.  Thanks for the link although I still find using the stock as a cheek rest uncomfortable. How does the smaller stock compare?

I had not thought about the blowback system being the limiting factor, but it makes sense.  I don't have access to a chronograph but I am very curious what velocity my varying loads are producing.  Ideally I want to work up loads and test them in a stock glock, 6" lone wolf, and the mech tech to see which powders produce the highest percentage increase. I would assume that a slower powder like AA #9 would benefit the most. I did notice something interesting when using the different powders and charge weights. 800x produced a much more mild recoil, even at 100% of book max. Also, the action appeared to operate smoother. For instance the other powders produced a sharp "snappy" recoil when fired, but the 800x seemed much more dull. Has anyone noticed this with other firearms?

I have the same setup but with the other M-4 stock.  I don't think its very comfortable to shoot with when your putting hundreds of rounds downrange.  I was looking for a buttpad, but might just buy a Magpul stock instead.  They have a pad and the cheek rest looks to be much more comfortable.

I received my Mech Tech last Friday for a glock20 10mm.  I put together around 10 different loads over the weekend using new Starline Brass, 180 grain HP and 200 grain Hardcast. I tried AA#9, Power Pistol, 800X, and Longshot ranging from 85% to 100% of book max. Noticed signs of overpressure at 100% of max. With the exception of Longshot, book max loads are relatively conservative to those used by Double Tap, Buffalo Bore, and Underwood. I had a near case seperation at max with Longshot (9.4 grains with a 180 grain HP).  All other powders at 100% of max had bulges at the case head.  I think I have reached maximum pressure limits for my particular Mech Tech. So far I am pleased with its performance, but a little bummed I can't comfortably match the loadings of Underwood and others.

Also my concern is that I had three Underwood rounds which experienced complete case seperation.  Looking at the Mech Tech chamber I can see a portion of the case is not supported, similar to the factory glock barrel, but now I'm wondering if the headspace dimension of the Mech Tech is correct.  Why else would I have case seperation from the same ammo in my Mech Tech, but never in a stock glock barrel?  Any ideas or suggestions?  Thanks