10mm...too hotly loaded?

It’s blowback, so no locking surfaces. Fired factory loads measure .425” at the case head, with 9.0 grains of Longshot I’m measuring.430” on average and with the 9.2’s I’m seeing.435” at the bulge in the case.

As an addendum, I started pulling the reloads down and I’m pretty sure that I must’ve crimped them too tightly, which could be causing my issue.
 
hatman1793 said:
Bashed flat & cratered primers, & a somewhat serious bulge. It’s a steamy load to be sure, not sure what you are trying to achieve with that FPS.....
IMHO, looks like either your bolt is unlocking too soon, or your barrel is somewhat unsupported near the cartridge case head.
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That FPS is not unreasonable for a 10mm carbine. The problem is its a simple blow back design, and it would seem pretty evident that there not enough mass in the bolt/carrier to keep things closed long enough to let the pressures drop.

There is a reason that handguns tend to use locked breeches for anything 9mm and bigger, blowback designs require too much weight. Rifles can use blowback with 9mm because they can use a heavier bolt, but clearly the TD isn't using a heavy enough bolt (and/or spring) for a hot 10mm.
 
Anyone who questions the velocity must not be familiar with 10mm rifles. I have at least three powders off the top of my head that will push a 180grn bullet past 1700fps within published data.

The TNW handles 10mm like a champ. My cases would come out with almost no case head expansion whatsoever and ejection popped them all out randomly about 6'. My previous photo shows the contrast with the first generation TD rifles...
 
B said:
Anyone who questions the velocity must not be familiar with 10mm rifles. I have at least three powders off the top of my head that will push a 180grn bullet past 1700fps within published data.

The TNW handles 10mm like a champ. My cases would come out with almost no case head expansion whatsoever and ejection popped them all out randomly about 6'. My previous photo shows the contrast with the first generation TD rifles...
Click to expand...

What do you suspect is the big difference between the TNW and the TD then? Bolt and/or spring weight?
 
I think Suther has touched on a possible cause. In looking through 10mm reloading receipes, the Hodgdon 2020 basic reloading manual has no 10mm loadings. However the 2019 Hodhdon manual shows loadings using LongShot powder.
If/should the bolt spring & bolt weight be insufficient to keep the breech closed, it could open prematurely......
 
Suther said:
What do you suspect is the big difference between the TNW and the TD then? Bolt and/or spring weight?
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The TNW bolt is massive and weighs well over a full pound. Even with an H3 buffer the TD bolt and buffer wasn't a single pound.

As for Hodgdon and Longshot, they have had data since I started using Longshot for 10mm in 2014. I was a big Blue Dot fan before that. Longshot is hands down the best 10mm powder if you seek maximum velocity.
 
I got a response from Thureon:

As you can imagine, for liability reasons I cannot recommend you load anything above SAAMI specs - not to say that you are - but since the gun is a blow back design and does not "lock up" like a traditional rifle this will be a challenge with hot loads. I am not aware if a heavier spring/weight combo would work and really hesitate to recommend altering the gun too much for your safety.
Sorry for the non-answer answer, but I am sure you can appreciate where we are coming from.
Thanks!
Fritz

Kind of a bummer about the non answer.
 
It's to bad you can adjust the dwell time on these blow back action rifles.. love my T.D.
Long shot is a great powder, your load matchs my data, little hard on brass, filling swipe marks
 
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My Thureon 10mm carbine had a slightly different problem which was, I think due to the blowback arrangement. I loaded up some 10mm with 800X powder close to maximum (but not exceeding). The reloads worked fine in a Glock 20. In the Thureon, I got case separation. The case ejected, but left a "wedding ring" inside the chamber. Cut off a piece of brass clean as a whistle and it looked like a wedding band when I picked it out. Lower power and factory worked OK. I don't think you can use full-house 10mm reloads with the Thureon.
 
That’s super disappointing. I love the rifle, but it’s sad that it’s not able to let you get the full potential of the cartridge. I may just load everything at 8.9 or 9.0 grains and hope that 1400 fps is adequate for 100m deer.
 
The engineer I dealt with moved on or was fired years ago from what I recall. Sort of sounds like whoever is there now doesn't know or care about the issue. Right before I had sold both my 10mm TDs I had them running balls out max loads with no issue. Leaves me wondering what happened to the company during and after the legal issues.
 
I can't say for sure this is the final version of the bolt/buffer assembly but it would be close. Solid steel buffer and a bolt with steel weights stacked like an AR buffer to prevent bouncing upon loading a new cartridge into the chamber. The rubber pieces are to soften the blow to the back of the buffer tube. This combo would be about 3x heavier than you could get with the regular bolt and any AR buffer on the market.

 
hifiwasabi said:
I got a response from Thureon:

As you can imagine, for liability reasons I cannot recommend you load anything above SAAMI specs - not to say that you are - but since the gun is a blow back design and does not "lock up" like a traditional rifle this will be a challenge with hot loads. I am not aware if a heavier spring/weight combo would work and really hesitate to recommend altering the gun too much for your safety.
Sorry for the non-answer answer, but I am sure you can appreciate where we are coming from.
Thanks!
Fritz

Kind of a bummer about the non answer.
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I know they say its a non-answer, but is it really? That kinda reads like they are telling you exactly the issue - their blowback system isn't tuned to handle hot loads - and then telling you its a non-answer to deflect from the fact that they are aware their system is flawed but they aren't interested in doing anything to try and make it better.

There are only two things that increase how long the bolt takes to open up in a blowback system, the mass of the bolt and the force of the spring. Heavier bolt makes the whole gun heavier, more forceful spring makes it harder to manually cycle the action and changes how the gun cycles/returns to battery. Makes me wonder if they had issues getting it to run reliably at both ends of the 10mm spectrum (mild factory ammo vs full power hand loads), and decided that functioning well with factory ammo was the better option?

I know the market for 10mm carbines is not very deep, but does anyone make a locked breech 10mm carbine?
 
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