Like most of us here we have purchased old retro barrels only to find out while externally they may look great and the crown looks OK they end up grouping like crap.  While I've had the MW gauge for a tad I recently picked up a throat erosion gauge. The first thing I did was to compare the barrel on my 604 clone (M VP 12) to the barrel on my 602 clone (VP 12).  I used the VP 12 as a guide because before I received it she was only proof fired and would be a good guide.  Here are some pictures of the gauges and how they were used.

These first three are of the gauges and plate instructions.




These two are of the MW gauge being used. There is a reference on the gauge for using it with the flash hider either on or off. I took the FH off the 602 because I thought the grenade retention band would throw off the results. The VP 12 barrel wouldn't accept the gauge into the barrel but the M VP 12 let about 1.5 millimeters in.



These last two are of the throat gauge being used. I'm assuming I'm using them correctly. The top one is of the 602 where the pre-embarkation line is even with the recess for the bolt cam pin. The bottom one is the 604 where one can see the pre-embarkation line is a bit further in.  



Over all I'm pleased with the M VP 12 of my 604. I was a bit worried before the test and I'm surprised the throat erosion isn't closer to the wear of the muzzle but she still shoots great.  Whoever had the barrel before me shot her and clearly knew how to clean it properly.  Maybe later I'll check the barrel of my Hydra-Matic.  I'd highly recommend getting these especially if one doesn't like rolling the dice with lady luck.

The throat erosion gauge...you did actually insert it into the chamber until it bottomed out...right?

Do you have any go/no-go/field chamber gauges?

I use the go/ no go gauges on my builds.

Yep.........the throat gauges bottomed out on both. Does something appear wrong?  I do not have any go/no gauges either.

Nothing at first glance. I'd still check headspace if I were you.  It's a pretty critical dimension.

For what it's worth I saw an 11.5" colt barrel shoot respectable groups despite darn near swallowing a throat erosion gauge.  Don't know if it was some sort of unique fluke or what but a 5 shoot group from an excellent shooting position stayed in a coffee cup sized circle at 100 yards with an aimpoint.

I'm confused with what you are saying because headspace has nothing to do with how a barrel measures with a TE gauge.  I've seen Garand barrels that measure a 7 on the TE gauge headspace just fine.  Also the spent casings from both these don't exhibit cause for worry.

The headspace suggestion was not directly related to TE.  I was simply saying that if you are going through the effort to check TE ME and the crown, I'd check the chamber specs as well.

Correct you are and sorry for the misunderstanding.  Those are the next tools I'll purchase.

RE: throat erosion...

Look up into the upper using the front edge of the magwell/feedramp area as your reference line.

Will do......thanks.