FINALLY - 5 Underwood 10mm Ammo Disections

[Kwesi]

It was easy to determine which cases were UW and fired in the CA89-10 vs the G20 by comparing the firing pin marks on the primers.  Every case has a case bulge (smiley?)!  FWIW: I do not recall having these with my personal 180 gr FMJ loads.  Anyways these are the photo's you requested:

[4949shooter]

Thanks Kwesi. From what I can make out in the picture, the striations in your Underwood brass don't seem to look any different than the spent brass from the thousands of rounds I had fired through my then issued P7M8.

I don't like those bulges, by the way.

[Yondering]

Is it just soot, or does that case second from the bottom (first picture) have a crack?

[REDLINE]

Wow.  Thanks very much for posting those pics.  Definitely looks like a number of those were super close to doing the same as the one that did rupture.

Sure does appear from the casings that your platform has a relatively generous chamber size, maybe even with quite a bit of the case left unsupported at the case head?

Does the bolt face enshroud the case head?  I guess I'm very curious to see how others discern what we see in those photos.  It's just not a platform type that I really have any real knowledge of.

[4949shooter]

It looks like the second round down from the top has a bulge in the same area as what appears to be the crack in the other case.

[REDLINE]

Is it just soot, or does that case second from the bottom (first picture) have a crack?

It does appear that that case was going to split similarly to the one in the original posted pics.

[Kwesi]

Is it just soot, or does that case second from the bottom (first picture) have a crack?

I thought it was soot BUT upon closer inspection it IS a CRACK and it goes all the way around, a full 360*!

[Kwesi]

Is it just soot, or does that case second from the bottom (first picture) have a crack?

It does appear that that case was going to split similarly to the one in the original posted pics.

You are right!

[Kwesi]

Thanks Kwesi. From what I can make out in the picture, the striations in your Underwood brass don't seem to look any different than the spent brass from the thousands of rounds I had fired through my then issued P7M8.

I don't like those bulges, by the way.

Why the bulges?

[4949shooter]

Thanks Kwesi. From what I can make out in the picture, the striations in your Underwood brass don't seem to look any different than the spent brass from the thousands of rounds I had fired through my then issued P7M8.

I don't like those bulges, by the way.

Why the bulges?

Redline, Yondering, and the others would know more than me, but the bulges are a sign of excess pressure and / or lack of chamber support. It's a mystery as to why you have a bulge in the center of your brass though, as opposed to the bottom end where the chamber support would drop off.

[Kwesi]

Does the bolt face enshroud the case head?

Not sure I understand.  If you google HK roller locked weapons you will find that the bolt head remains locked until the roller on each side retracts (safe pressure) and allows the bolt to travel rearward.

[Kwesi]

Thanks Kwesi. From what I can make out in the picture, the striations in your Underwood brass don't seem to look any different than the spent brass from the thousands of rounds I had fired through my then issued P7M8.

I don't like those bulges, by the way.

Why the bulges?

Redline, Yondering, and the others would know more than me, but the bulges are a sign of excess pressure and / or lack of chamber support. It's a mystery as to why you have a bulge in the center of your brass though, as opposed to the bottom end where the chamber support would drop off.

Understood but the pressure must be much higher since I never have the bulge with my loads.

[REDLINE]

Well I don't know if this is possible, not being familiar with the weapon and all, but can you take a pic or two of how the rounds sits against or into the bolt face, and separately a round sitting in the chamber without the bolt up against it?

That would help me understand further a great deal.  If not, that's cool too.

[Intercooler]

Lack of support. Throw a round in that puppy and take photo for us. Looks like the last 1/4" is fully unsupported.

[REDLINE]

If you google HK roller locked weapons you will find that the bolt head remains locked until the roller on each side retracts (safe pressure) and allows the bolt to travel rearward.

I'll have to google that and come back with a better understanding.