Guys, have anyone herd of a 8mm Throat Erosion Gauge? With all of the surplus MG42/53 barrels out there. I need a tool to check how good the barrels are. I have several, but the only way I have is to check them is to clean and fire them. It would sure save time and ammo if there was a way to check with out the time it the range. Does anyone know where I can get one? I have built post sample MG42/53s for several years, and I have never took the time to get one.
Thanks, P. Stout 07FFL/SOT
8mm Throat Erosion Gauge
Re: 8mm Throat Erosion Gauger
I have not seen one for 7.92x57JS but have one in .303. I can measure it and scale it for 7.92. As the original caliber gauge was for 7.94 (Bore Measurement) reject it should work out. The gauge I have is in "Grand Old Lady" if you want to look at it.
Let me know and I will dig it out
Chuckm
Carson City, Nevada
Let me know and I will dig it out
Chuckm
Carson City, Nevada
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JBaum
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Re: 8mm Throat Erosion Gauge
The original sets are big bucks when you can find them (maybe once a year). I've seen a full set sell for $1,100. I'm sure you're not looking for a collectible set though.
The German set used different threaded cylinders (13, I think) to attach to the end of a rod, which was then pushed into the barrel to the prescribed distance. Each little cylinder was one step bigger than the last, and if the bore got to a certain size, it was considered worn out and replaced. Details are on page 60 section b of the MG42 armorer's manual.
The German set used different threaded cylinders (13, I think) to attach to the end of a rod, which was then pushed into the barrel to the prescribed distance. Each little cylinder was one step bigger than the last, and if the bore got to a certain size, it was considered worn out and replaced. Details are on page 60 section b of the MG42 armorer's manual.
Re: 8mm Throat Erosion Gauge
pdstout and John:
I had read the manual (First,Really!!) but thought that the one for the .303 and the U.S. .30 cal was a better (Read quicker!) way to go. The rod is about 8-9 inches long and is graduated in inches on the British one. Shaft is tapered and when inserted into the chamber end you read the mark at the breach face. The measurement shown on the rod is a measure of how many rounds of life left before the accuracy is to bad to give over-head fire support.
I will measure mine in the morning and as i have a unfired Vickers barrel, will let you know what the difference is for each stage. Then we can work out what is should be for 7.92mm
Chuckm
Carson City, Nevada
I had read the manual (First,Really!!) but thought that the one for the .303 and the U.S. .30 cal was a better (Read quicker!) way to go. The rod is about 8-9 inches long and is graduated in inches on the British one. Shaft is tapered and when inserted into the chamber end you read the mark at the breach face. The measurement shown on the rod is a measure of how many rounds of life left before the accuracy is to bad to give over-head fire support.
I will measure mine in the morning and as i have a unfired Vickers barrel, will let you know what the difference is for each stage. Then we can work out what is should be for 7.92mm
Chuckm
Carson City, Nevada
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JBaum
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Re: 8mm Throat Erosion Gauge
The German model didn't measure throat erosion near the chamber, nor muzzle wear, it was pushed into the barrel (9" seems to ring a bell), and measured the wear of the center area of the barrel.
All are indicators of the same thing, the useful life of the barrel.
All are indicators of the same thing, the useful life of the barrel.
Re: 8mm Throat Erosion Gauge
I'm surprised to see that no one mentioned you could make a chamber casting and measure the casting. I have made a few out of the Cerrosafe alloy, which is reusable, but I did just as well with the sulfur casting. You would need to look in a reloading book or perhaps a reamer diagram to know how deep the rifling grooves should be before they engage the bullet. You'd be able to measure that directly from the casting to know how much off you were.
Safety tip - the sulfur casting could be real messy, real fast, if you don't use plenty of ventilation and think about what you're doing. Most people with the skill to build a semi MG42 clone have enough grey matter to be safe with sulfur and an open flame, but not everyone may have grown up with a chemistry set to know that sulfur will burn.
If you find the throat is eroded slightly, the typical solution is to run a reamer in deeper to compensate, then re-set headspace. That is for a normal rifle. I would think that on an MG42 design, it would still be doable up to the point that you run out of barrel to seat in the bushing.
Another thought - I've no idea what barrel life expectancy is on an MG42, but heating the barrel less with long bursts, and using ammo with cooler-burning powders is recommended to maximize barrel life. The cheap-o military surplus is almost always loaded pretty hot. Food for thought.
Safety tip - the sulfur casting could be real messy, real fast, if you don't use plenty of ventilation and think about what you're doing. Most people with the skill to build a semi MG42 clone have enough grey matter to be safe with sulfur and an open flame, but not everyone may have grown up with a chemistry set to know that sulfur will burn.
If you find the throat is eroded slightly, the typical solution is to run a reamer in deeper to compensate, then re-set headspace. That is for a normal rifle. I would think that on an MG42 design, it would still be doable up to the point that you run out of barrel to seat in the bushing.
Another thought - I've no idea what barrel life expectancy is on an MG42, but heating the barrel less with long bursts, and using ammo with cooler-burning powders is recommended to maximize barrel life. The cheap-o military surplus is almost always loaded pretty hot. Food for thought.
Re: 8mm Throat Erosion Gauge
Well I finally got a chance to do the measurements for the .303 erosion gauge.
It assumes that, for 7.92mm you WILL change barrels every 150 rounds in full auto with 5-7 shot bursts, and for .303 burst length will be 25 rounds.
for the .303 SM75 Barrel Erosion Gauge for Gun, Machine, .303, Vickers:
6 inch long round stock Nominal 3/63" diameter to start with.
Front 1.5" section ground to .305 inch (chamber end)
Rest of shaft turned to .290" nominal
Grind flat section starting 2" from start of .305" section with a depth of .110"
First mark is 2.75" from chamber end, then marked every 1/4" up 4.50"
Bore diameter is .303 nominal with a grove diameter of .010 for a total of .313 nominal
To use, put gauge into chamber and read the measurement at face of chamber
Reject for .303 was gauge entering to 3.50" at face of chamber for use a firing over the heads of own troops. Barrel was condemned at 4.50" for any use.
7.92x57JS has a bore of 7.92mm and a grove of .028mm for a total of 8.20mm bullet diameter.
Reject gauge was 7.94mm originally but was rescinded in 1942 (?) as to tight for combat.
You would need to measure a known, unfired barrel and see what the distance from the chamber cone to the face of the chamber was. Then make up a rod and grind the front section of it to 7.93-7.94mm dimension for about 25mm. Insert it in the barrel and from the face of the chamber, mark it every 6.3mm. This would give you a start as to the amount of erosion found after each thousand rounds fired.
The dimensions for the 7.92x57JS are approximations from a couple of sources an the bullet diameter and grove depth. I have not found and info on the MILITARY dimension specifications and did not use SAMMI spec's as our barrels were not made to them!!
I hope this helps, any questions just ask and I will try and answer
Chuckm
Carson City
Nevada
It assumes that, for 7.92mm you WILL change barrels every 150 rounds in full auto with 5-7 shot bursts, and for .303 burst length will be 25 rounds.
for the .303 SM75 Barrel Erosion Gauge for Gun, Machine, .303, Vickers:
6 inch long round stock Nominal 3/63" diameter to start with.
Front 1.5" section ground to .305 inch (chamber end)
Rest of shaft turned to .290" nominal
Grind flat section starting 2" from start of .305" section with a depth of .110"
First mark is 2.75" from chamber end, then marked every 1/4" up 4.50"
Bore diameter is .303 nominal with a grove diameter of .010 for a total of .313 nominal
To use, put gauge into chamber and read the measurement at face of chamber
Reject for .303 was gauge entering to 3.50" at face of chamber for use a firing over the heads of own troops. Barrel was condemned at 4.50" for any use.
7.92x57JS has a bore of 7.92mm and a grove of .028mm for a total of 8.20mm bullet diameter.
Reject gauge was 7.94mm originally but was rescinded in 1942 (?) as to tight for combat.
You would need to measure a known, unfired barrel and see what the distance from the chamber cone to the face of the chamber was. Then make up a rod and grind the front section of it to 7.93-7.94mm dimension for about 25mm. Insert it in the barrel and from the face of the chamber, mark it every 6.3mm. This would give you a start as to the amount of erosion found after each thousand rounds fired.
The dimensions for the 7.92x57JS are approximations from a couple of sources an the bullet diameter and grove depth. I have not found and info on the MILITARY dimension specifications and did not use SAMMI spec's as our barrels were not made to them!!
I hope this helps, any questions just ask and I will try and answer
Chuckm
Carson City
Nevada
Re: 8mm Throat Erosion Gauge
Well I have some more info on the .303 gauge, reject was 3.50 inches with 2.75 inches at the start being 7,000 rounds remaining and the markings are 1 tenth inch marks.
A new barrel was supposed to last for 10,000 to 12,000 rounds from when new, and the
Gunner was to maintain a "Rounds fired" sheet for each barrel. After 7,000 rounds it was a question of accuracy that condemned the barrel or reaching 3.50" which ever came first.
Chuckm
Carson City
Nevada
A new barrel was supposed to last for 10,000 to 12,000 rounds from when new, and the
Gunner was to maintain a "Rounds fired" sheet for each barrel. After 7,000 rounds it was a question of accuracy that condemned the barrel or reaching 3.50" which ever came first.
Chuckm
Carson City
Nevada
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JBaum
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Re: 8mm Throat Erosion Gauge
The flange near the threads on a rifle barrel was often marked with the actual measurement of the barrel, which may not have been 7.92 - I've seen them marked 7.94. The wear was then determined by the difference between the actual measurement and the stamping.
Re: 8mm Throat Erosion Gauge
John:
Thanks for the info, I guess you would have to "Slug" the bore with a lead slug and then see what the size of the ground end would be for that barrel. Now I see why the German kit came with the different gauge sizes. Made life a little easer!!
Chuckm
Carson City
Nevada
Thanks for the info, I guess you would have to "Slug" the bore with a lead slug and then see what the size of the ground end would be for that barrel. Now I see why the German kit came with the different gauge sizes. Made life a little easer!!
Chuckm
Carson City
Nevada