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headspacing comments

1276 Views 15 Replies 7 Participants Last post by  jreifsch80
hey all i'm by no means an expert as this post will show but let me take this time to let the veterens scold me and the newer guys learn from me. so i recently finished my m76 and i was too excited to do a function test so i head spaced with a round as my go (firing pin and trigger parts removed) and a round with masking tape as my no go. i got everything set and drilled and installed my barrel pin. i tested with both 12-53 yugo brass case and 70's romy steel (8x57) the first 10 rounds of yugo functioned flawlessly like a sewing machine, the next 20 rounds of steel functioned great too well here's where the problems started, i decided to try one more mag of yugo and i got a couple cracked cases with the 8th round having a case head seperation and of course i don't have an 8mm case extractor though i did carefully get it out with a screwdriver without harming my chamber.

so just this morning i tried again my ******* voodoo go no go method and it seemed within spec (how ever much it can be that way) but just a shade looser on the round than before. so my round as a go might have been setting the HS more towards the no go side of things since i wasn't using any more pressure to push the round in the chamber than my excited little hands could give the bolt. also my word of advise on a new barrel install is to set it for a slightly tight but functional chamber as i experienced my chamber opening up a little which i confirmed it was the barrel settling forward by a tiny amount of space that wasn't originally there between the receiver and the rear sight block.

i'm lucky i didn't get hurt and it wont be hard to fix my problem though i'll have to use an over sized barrel pin now.

I had no problems with the steel ammo at all so i was wondering if maybe my 1953 yugo brass might have just been brittle, well either way i need to do it right and order a set of gauges then if i am getting ruptured cases after properly setting my headspace then i know to save my 50's yugo for my mauser and just feed my m76 romy steel.

ok regulars, scold away :biggrin:
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Didn't you file your barrel pin groove?
No scolding here just a few thoughts. The case separation can happen to old ammo even with the correct headspace, maybe it was bad brass.. get a broken shell extractor for 8mm or 30-06..its a good tool to own.. get a proper headspace gauge and recheck your gun before you go to the oversize pin. I would never use a loaded round as a headspace gauge as they can be out of spec, the yougo also is known to have a oversized bullet olgive, sometimes it needs to have the barrel throated to function in some barrels. also I believe the 8mm mauser has had some changes to the case dem. over the last 60 years and one case may be slightly different that the next.. I think ashoe also asked if you filed the barrel pin grove before drilling your barrel pin hole if you just drilled it with out removing the bulk of the barrel metal the rotation of the drill bit can move the headspace either tighter or looser depending on which side of the receiver your drilling from. clamp the barrel when drilling and not the receiver. The steel case romo ammo is really good stuff, it is my choice for 8mm but it is expensive these days.. AK are mostly designed around the steel case ammo but the yougo are the exception almost all yougo ammo is brass case.. I'm looking forward to finishing mine, but I'll need to order headspace gauges as well. B2B
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I would get a real headspace gauge because you want to check it after a few round anyway to see if there is a lot of slop produced as you shoot the gun. If it is a Ohio Rapid Fire receiver check the locking lugs they could be fine now but I would keep checking them for the life of the rifle. I built a virgin M72 kit on one of their milled receivers with the head space perfect and after about a hundred both the locking lugs bent back. They refunded my money and were nice but it was a lot of time wasted and dangerous.
thanks for the advice guys. b2b, you're right those yugo brass cases are a hair longer from the base to the shoulder though i have no way of measuring exactly, but i know the headspace stretched from the distance of gap (.010) between my rear sight block and receiver, also my steel cases that gave me no problems have the primers pushed out the same amount now. let me clarify how i did my barrel pin as i might be confusing people i said reamed it just for simplicity but i was incorrect, i drilled it with the largest bit that would fit in the receiver pin hole and went insainly slow with cutting oil basicly chipping barrel matterial away, i wasn't worried about my drill press moving the barrel as i went super slow and i broke my press pressing in my barrel at first, it's a tight fit but once i rewelded my press it pressed in without too much tension and i was super carefull to not bow let alone bend my barrel. then after that i used round and oval jewelers files to file the drilled barrel to exactly same hole size as the receiver, my barrel pin pressed in just fine. the way i was drilling if my barrel would have moved, it would have gone farter in the receiver at first and the half way i switched sides and the other side it would have moved out so who knows but either way after i pinned it the rear sight base was still tight against my receiver, i assume the barrel moved out .003 (edit i had .010 creep out but that was how much the primer was pushed out the sight block is only .030 away from the receiver) after maybe 20 rounds, not sure if it would creep out more but i doubt it.

dark knight, i did check my lugs and i actually filed them with jewlers files (equally) to square them with the bolt and eventually the breech confirmed this. i assume the lugs not being square would cause headspace to grow if not squared before headspacing and firing as the pressure from firing would probably break them in and eventually hammer down the higher points a bit so that might be why people have to readjust their headspace on their orf receivers, not to mention if they didn't have a proper heat treat but that's another issue i hopefully don't have (luckly i have good heat treat places here just in case but if it's a bad heat treat by the time i notice it it might be too late) just my thought. Sounds like the heat treat on the last run of receivers is proper from what i've read but i'm still making a spare m76 reciever from my m70 front stub anyway, maybe i'll make an m76 in .308 with a spare psl carrier i have, but anyway i would have sent back the receiver if I would have paid their original price which i think was 499 but for 200 bucks i don't mind the little adjustments i had to make too much.

I am getting a headspace gauge set either way, but i'll check it before drilling
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so i head spaced with a round as my go (firing pin and trigger parts removed) and a round with masking tape as my no go.
well ill scold a little. you took a round that may or may not with in speck and used a compressable substance to set clearance on some thing that holds around 55,00 PSI or more. (did you measure the tape??) These recivers are questionable IMOO to begin with and you have no reall good way to determine if the lugs have moved or the barrel has.

on my calbure conversions I take a measurement from the go guage to the back of the trunion and record it that way if something moves I can determine if the barrel moved or the lugs are moving or wearing rapidly.

this stuff is serious and it needs to be done correctly. unknown dimension rounds and masking tape are not part of doing it correctly. It seems we are seeing more and more cobbling methods on the net lately. some of the Utube crap is down right scary.

I doubt the ammo is bad but it cant be ruled out. for all we know it was scraped because it was wrong and then sold to you.

Im glad you didnt get hurt an the gun didnt get destroyed.

measure some fired cases and see how much they expanded over a unfired round if its a head space issue it should show up with a gase that is streched to far.
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i completly agree biggun, we need to hear more about goofs people make when building to have more proof why not to do certain short cuts. in fact the little voice in my head kept telling me not to do it but my excitment got the better of me, you're totally right that i am lucky i didn't get hurt not hurt my rifle, also i was too excited that i even forgot to measure my tape so everything i could have done wrong i did it. also not sure if the lugs moved but i know for sure the barrel moved forward .030 since for the rear sight block to have moved forward at all would have either made my gas tube too tight to get off (it was nice and snug like it was when i fit it that way) or it would have had to have move the gas block forwar too which i know it didn't. when i HS it properly i am going to take measurements from the no go to the face of the lugs and keep them where i can find them. as far as stretching cases i know it did that as i have a yugo case that didn't crack and it is a little longer than an unfired one i think before the barrel setteled forward .030 i might have ben at the very edge of cracking the brass and then i probably finished setteling with my steel ammo (which didn't stretch of rupture by the way) i personally think there isn't anything wrong with my brass 50's yugo ammo as it functions flawless in my m48 (though it's a very nice tight rifle that loves anything you feed it) but semis are a different animal. on a lighter note on the first run with 50's yugo bofore my problem my m76 seemed to like it no crashing of the carrier in fact ocassionally wouldn't cycle all the way on gas setting 1 but gas setting 2 worked like a charm, same with my romy steel case ammo.


tomorrow i am either buying locally if i can or ordering a go and no go, for you other guys i've been talking to who are building m76s, if you're low on funds and don't have gauges talk to me and we can work out you barrowing my gauges if need be
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If you think you'd only need it for a one time thing, you might want to check with a local gun shop or smith to see if they would rent you a set or just have them check it while you wait.
Could save a few bucks?
yeah that's true though i want to rebarrel my m48 down the road so i'm not against owning a set plus being an orf reciever i might want to periodicly check my headspace just in case. i'll probably try that if i build a psl down the road though
I may get blasted for this but here goes. If you only have the funds ( aka Cheap Bastard like me) just buy a GO GAGE. I only own two complete three gage sets with GO, NOGO, and FIELD gages. You can shim a GO with a .004 to equal a NOGO and .008 (.010 in some cases) for FIELD. Most of my sets have only a GO and NOGO. A lot of people have this idea if a NOGO goes there is a problem. GO and NOGO are typically used for initial setting of headspace. If FIELD goes, this is the true REJECT condition. I had a Garand that was between GO and NOGO specs have a casehead seperation. I pulled the trigger and it vented into the mag well and split the stock in half. Next thing I was holding a THREE piece Garand. The problem was early 50s surplus British Kynoch ammo with brittle brass. Some may not like my idea of shim improvise but bet all will agree it's better than using shell casings. HTH willie
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Nothing wrong with a metal shim on a correct guage. its the use of unknown ammo and unknown shim dimensiuons like tape that cause problems. I neecd to find the post were i measured a bunch of differant masking tapes. seems like two layers could be over .010

also not sure if the lugs moved but i know for sure the barrel moved forward .030
If the barrel moved forwrd thirty thousands you got some reall issues with your barrel pin set up. you barrel should not move at all. I can see perhaps .0005 due to lughs seating in and a slight amount of take up on the pin but no way should any thng be moving you likey now have close to .040head space when it should be .004
that was a typo biggun i'm sorry i mean .003 is what the barrel moved but as we were talking about before that isn't confirmed by measuring off the lugs that's how much space opened up between the receiver and the rear sight measured with a feeler gauge. it's totally possible my rear sight block moved a tiny bit also but the gas tube wasn't any tighter than it was before i went to the range. like i said earlier it probably orriginally head spaced and the very long edge as i had no problems with my first 10 rounds of yugo then had no problems what so ever same with the next 20 rounds of steel romy then the last 10 rounds of brass yugo gave me my problems i believe i had 3 slightly cracked cases (found out from inspecting bass after my misshap) and then my 8th round of the mag gave me the full head seperation. I just compared the the length of the another box of unfired rounds from the same batch and compared them to last couple fired brass (tiny cracks at the base). i measured overall case length and the unfrired brass were all within 2.238-2.240, the cases i have from the final magazine all ranged from 2.250-2.255, closer to the 2.25 were the non cracked cases from that magazine and the longer ones were cases with very slight cracks, i only noticed the cracks when i was inspecting brass from the last mag after my seperation and i just noticed a tiny little black line and on further examination i can see a little crack. i ordered a go and no go today from brownels with a friend's order who was ordering some other stuff as well so should come by the end of the week, i'm making an educated guess that headspace could be somewhere around .015. luckly the same guy that ordered my go no go set is going to let me come to his shop saturday and either him or welldog, who are both skilled machinists, are going to help me over size oversize my barrel pin hole and make a new pin. obviously after setting proper headspace.
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you could also weld up the slot on your barrel and re cut it, that is another option to the oversized pin, which pin did you use btw..the kits don't come with them. I'd rather use a standard pin if I had a choice. B2B
i used an m70 pin i had started it with a light hammer and it was tight but it pressed in easily. you know i though of that too but i didn't like the idea of welding on my chamber and heating it up, or could a guy keep his heat down enough with a tig? either way i would rather rather oversize it i was thinking with a mill cutting the hole and oversizing just in the direction i need to oversize so if for instance say i move my barrel back .020 (just picked a round number for example) instead of reaming the entire hole and installing a .040 over pin i'll see if we can cut the hole only .020 over basicly moving the center of the barrel pin straight back .020 and the top and bottom of the hole i would assume would be oversize from center .010 with the front of the hole pretty much keeping the original front of the hole.

is that how a guy normally over sizes his barrel pin hole? or does he keep his original center? actually just thinking about it now it would probably be easier to keep the original pin location and if i move the barrel back say .020 bore it .040 over. i dunno i'll see what the machinists think
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Im not a big fan of welding up any thing on a barrel especaly on top of the chamber.

to put it simple you have to reset the barrel and then drill the hole untill it cleans up. and then fit a pin to the hole.
i was thinking along the same lines biggun thanks for all the help guys, if i get to it this saturday i'll let you know how it goes
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