Mk12 won’t feed reloads

[Daddyhorn]

Mk12 won’t feed and lock my hand loads. I have Douglas barrel, standard mil bolt carrier with a jp bolt. Hand loads are 77gr smk loaded into surplus standard mil brass. OAL is 2.25. Measured the case and everything seems to be within standards. Same rounds operate fine through my buddy’s rifle with a Daniel defense barrel. I know the tolerances on my barrel should be tighter. Bolt locks if rounds are manually loaded individually but will not lock if a mag is used. Also if loaded manually they are hard to eject manually. I’ve tried brand new p mags as well as the standard garbage ones. Also swapped the bolt carrier already. Must be something in my hand loads that I’m not seeing. Using RCBS small base two die set with Lee single stage press. Appreciate the help.

 

[6.5SH]

Do you have a comparator or Wilson style case gauge?

 

[Daddyhorn]

No i do not.

 

[nn8734]

What is your casehead-to-shoulder datum measurement on your sized brass (before firing)?

 

[6.5SH]

If you have a empty 9mm case you can use it with calipers as a ersatz comparator.

 

[Daddyhorn]

Oh boy. I’m going to regret this. How do you find the datum point? Pretty new to reloading (obviously at this point). Only measurement tools I have are a micrometer

 

[Daddyhorn]

If you have a empty 9mm case you can use it with calipers as a ersatz comparator.

I have one

 

[nn8734]

Micrometer (assuming it’s a 1” micrometer) is useful for measuring case head expansion just above the extractor groove to determine if your at or beyond safe pressure thresholds. Measure the case before and after firing.

Also purchase a Hornady headspace gauge tool with the set of bushings and watch a couple vids on YouTube to learn how to use them.

 

[Daddyhorn]

Micrometer (assuming it’s a 1” micrometer) is useful for measuring case head expansion just above the extractor groove to determine if your at or beyond safe pressure thresholds. Measure the case before and after firing.

Also purchase a Hornady headspace gauge tool with the set of bushings and watch a couple vids on YouTube to learn how to use them.

Ok. I’ll go ahead and order one of those. Factory loads work fine. Just mine. They group great just won’t load and it’s frustrating.

 

[Skunk]

Are you full-length sizing your brass, or just the necks?

 

[6.5SH]

Oh boy. I’m going to regret this. How do you find the datum point? Pretty new to reloading (obviously at this point). Only measurement tools I have are a micrometer

I assume you have calipers as well since you provided a COAL measurement.

Place the open mouth of the 9mm case over the 5.56 case neck against the shoulder. Get the bases of both cases as flat as possible against the jaws of your calipers, it can take some fiddling with but you'll get the feel for it.

Note the measurement. Compare the difference between fired and sized brass. At minimum the sized piece should be .002" smaller, but some semi-auto chambers want closer to .004-.005" to be happy.

 

[Daddyhorn]

Are you full-length sizing your brass, or just the necks?

Full length

 

[nn8734]

Ok. I’ll go ahead and order one of those. Factory loads work fine. Just mine. They group great just won’t load and it’s frustrating.

You will need bushing A330. Without that tool we won’t know if the sizing die is set up correctly.

Here is how it’s used

 

[Daddyhorn]

You will need bushing A330. Without that tool we won’t know if the sizing die is set up correctly.

Here is how it’s usedView attachment 7670507

I’ll go see if my local store has one. If not I will get one ordered

 

[nn8734]

I’ll go see if my local store has one. If not I will get one ordered

I also have a Mk12 (mod 1 / mod H) and set my bump to 1.457 as fired brass measures 1.461-1.462. Barrel is also a Douglas 1/7 SPR contract barrel from PRI.

For comparison, my carbine’s fired brass measure 1.465-1.466 on average.

the piece of brass in the pic hasn’t been sized as of yet.

 

[mtrmn]

If you are crimping try loading a few rounds without the crimp. If they work where the others do not, then you are over-crimping, most likely with your seating die.

ETA: If you want to crimp your rounds, do it in a seperate operation. Do not try to crimp and seat the bullet at the same time. Get a Lee factory crimp die and crimp all your rounds after the bullets have been seated.

 

[supercorndogs]

You can use a 9mm case instead as a comparator. To get a base to shoulder to measurement. Then you will have to surmise if it is sticking at the shoulder or the body. You could color one with a sharpie and see where it s sticking. You will either need to bump the shoulder more, or possibly you will need a small base die.

 

[Daddyhorn]

If you are crimping try loading a few rounds without the crimp. If they work where the others do not, then you are over-crimping, most likely with your seating die.

ETA: If you want to crimp your rounds, do it in a seperate operation. Do not try to crimp and seat the bullet at the same time. Get a Lee factory crimp die and crimp all your rounds after the bullets have been

I assume you have calipers as well since you provided a COAL measurement.

Place the open mouth of the 9mm case over the 5.56 case neck against the shoulder. Get the bases of both cases as flat as possible against the jaws of your calipers, it can take some fiddling with but you'll get the feel for it.

Note the measurement. Compare the difference between fired and sized brass. At minimum the sized piece should be .002" smaller, but some semi-auto chambers want closer to .004-.005" to be happy.

my sized brass starts going into the spent 9mm.

 

[6.5SH]

You can try resizing the 9mm brass and see if that shrinks it down enough for you. If not just wait for your comparator to arrive.

 

[mtrmn]

After a loading session, I drop every round into one of these JP case gauges. If it fits the gauge it fits your chamber.

JP Rifles

 

[Daddyhorn]

After a loading session, I drop every round into one of these JP case gauges. If it fits the gauge it fits your chamber.

JP Rifles

Will that let me know what adjustments to make if it doesn’t fit or will it just not fit?

 

[mtrmn]

It won't fit. Are you crimping your rounds?

 

[Daddyhorn]

I don’t have the gauge yet. Was just asking. My local store was closed tonight by the time I got there so I will try again tomorrow. Thank you all for the help.

It won't fit. Are you crimping your rounds?

 

[mtrmn]

I don’t have the gauge yet. Was just asking. My local store was closed tonight by the time I got there so I will try again tomorrow. Thank you all for the help.

Are you crimping the bullets in the case? If so, are you using the roll crimp feature in the seating die to do this?

 

[Daddyhorn]

Are you crimping the bullets in the case? If so, are you using the roll crimp feature in the seating die to do this?

Yes and yes

 

[mtrmn]

Yes and yes

Take a couple of your KNOWN BAD rounds and hold them up to the light. Looking at the silhouette profile of the case, look just below the point of the shoulder. Can you see a slight bulge at or just below that point of the shoulder? If so, that's your problem and the rounds are being over-crimped causing the shoulder to collapse.
ETA: you can use a straightedge against the case to help determine if it is bulged.

 

[alamo5000]

Mk12 won’t feed and lock my hand loads. I have Douglas barrel, standard mil bolt carrier with a jp bolt. Hand loads are 77gr smk loaded into surplus standard mil brass. OAL is 2.25. Measured the case and everything seems to be within standards. Same rounds operate fine through my buddy’s rifle with a Daniel defense barrel. I know the tolerances on my barrel should be tighter. Bolt locks if rounds are manually loaded individually but will not lock if a mag is used. Also if loaded manually they are hard to eject manually. I’ve tried brand new p mags as well as the standard garbage ones. Also swapped the bolt carrier already. Must be something in my hand loads that I’m not seeing. Using RCBS small base two die set with Lee single stage press. Appreciate the help.

Maybe I am just reading differently than others but I don't see your powder charge.

I also fail to see how you resize or anything else like that matters.

Your buddy might have a gun with a giant gas port whereas yours might not be. He could in that situation create a load that works great for him but not for you.

The buffer weight, gas port, adjustable gas block (or not), gas system length, all have a much more prominent role here, at least with the information that is given.

Borrowing someone else's load data is generally a very bad idea and this is just one more example of why that is true.

I other words I don't think this is entirely a reloading question.

 

[scharfschutze762]

Is it a home built MK12 or actual factory clone..And if its a home built please give the specs of the gas block and buffer tube..Rifle or carbine buffer tube and what buffer weight.

 

[mtrmn]

Maybe I am just reading differently than others but I don't see your powder charge.

I also fail to see how you resize or anything else like that matters.

Your buddy might have a gun with a giant gas port whereas yours might not be. He could in that situation create a load that works great for him but not for you.

The buffer weight, gas port, adjustable gas block (or not), gas system length, all have a much more prominent role here, at least with the information that is given.

Borrowing someone else's load data is generally a very bad idea and this is just one more example of why that is true.

I other words I don't think this is entirely a reloading question.

His reloads won't chamber due to being too tight

 

[alamo5000]

His reloads won't chamber due to being too tight

I must have missed that part. If they won't fit that can be numerous problems as well. If the die is set up wrong it will cause problems like that.

Same thing with super long brass. It might need to be trimmed.

What I would do is get a magic marker or equivalent and paint a piece of the brass with it then try to chamber it.

When you pull it out it will show you the contact points and help diagnose the problem.

 

[Daddyhorn]

I must have missed that part. If they won't fit that can be numerous problems as well. If the die is set up wrong it will cause problems like that.

Same thing with super long brass. It might need to be trimmed.

What I would do is get a magic marker or equivalent and paint a piece of the brass with it then try to chamber it.

When you pull it out it will show you the contact points and help diagnose the problem

I built the rifle. Douglas SPR barrel from pri. PRI adjustable gas block. Rifle length gas system and buffer. Loaded with 21.8 grains of H322. I’m an active duty marksmanship instructor and the barrel was installed by a spec war armorer. New to reloading though so I’m pretty sure that’s where I screwed up and it feeds all factory rounds fine. I’ll attach some pics that may help although I am going to buy the recommended gauge. The side by side rounds are factory 62 grain with the green tip. Other round is mine. 77 gr smk with a 2.25 OAL. Other pics are the marker trick. A before and after. I can’t tell if the missing marker are from the trip up the feed ramps or not. Maybe you guys will see something. Again I really appreciate the help. This is frustrating to say the least.

 

[FCS]

Mk12 won’t feed and lock my hand loads. I have Douglas barrel, standard mil bolt carrier with a jp bolt. Hand loads are 77gr smk loaded into surplus standard mil brass. OAL is 2.25. Measured the case and everything seems to be within standards. Same rounds operate fine through my buddy’s rifle with a Daniel defense barrel. I know the tolerances on my barrel should be tighter. Bolt locks if rounds are manually loaded individually but will not lock if a mag is used. Also if loaded manually they are hard to eject manually. I’ve tried brand new p mags as well as the standard garbage ones. Also swapped the bolt carrier already. Must be something in my hand loads that I’m not seeing. Using RCBS small base two die set with Lee single stage press. Appreciate the help.

In lieu of a comparator to see how much you are bumping shoulder...

Size a piece of fired brass as sizer die is currently set up, remove all lube, lock BCG back, drop case in chamber, ease bolt forward and use forward assist (FA) to get extractor to snap over rim, put bolt fully into battery.
IF you are unable to get bolt to go fully into battery or really have to apply a lot of force to get bolt fully into battery, case isn't sized enough.
IF bolt will go fully into battery but will not extract easily, case isn't sized enough.
IF bolt will go fully into battery and case extracts easily, you have case sized enough and problem is elsewhere.

Once you have a case that allows for bolt to be fully in battery, extracts easily, now make a dummy round by seating a bullet. DO NOT crimp, only seat bullet. Verify dummy round will chamber using same ease bolt forward until contacts dummy round and finish getting bolt fully into battery with the FA. Provided dummy round extracts easily, now crimp bullet and repeat chambering and extracting the dummy round.
Really is better to seat in one step, crimp in another separate step.
You are using 77 SMK with cannelure, correct?

I suspect brass isn't sized enough for your chamber and that is causing issues.

What powder?
What charge weight?
What brass?
What primer?

 

[Daddyhorn]

In lieu of a comparator to see how much you are bumping shoulder...

Size a piece of fired brass as sizer die is currently set up, remove all lube, lock BCG back, drop case in chamber, ease bolt forward and use forward assist (FA) to get extractor to snap over rim, put bolt fully into battery.
IF you are unable to get bolt to go fully into battery or really have to apply a lot of force to get bolt fully into battery, case isn't sized enough.
IF bolt will go fully into battery but will not extract easily, case isn't sized enough.
IF bolt will go fully into battery and case extracts easily, you have case sized enough and problem is elsewhere.

Once you have a case that allows for bolt to be fully in battery, extracts easily, now make a dummy round by seating a bullet. DO NOT crimp, only seat bullet. Verify dummy round will chamber using same ease bolt forward until contacts dummy round and finish getting bolt fully into battery with the FA. Provided dummy round extracts easily, now crimp bullet and repeat chambering and extracting the dummy round.
Really is better to seat in one step, crimp in another separate step.
You are using 77 SMK with cannelure, correct?

I suspect brass isn't sized enough for your chamber and that is causing issues.

What powder?
What charge weight?
What brass?
What primer?

I’ll try this tomorrow. I am using 77 smk without cannelure. I do very lightly crimp them as of now. 21.8 grains of H322 and Im using brass from standard mil spec 55 grain ball.

 

[FCS]

I built the rifle. Douglas SPR barrel from pri. PRI adjustable gas block. Rifle length gas system and buffer. Loaded with 21.8 grains of H322. I’m an active duty marksmanship instructor and the barrel was installed by a spec war armorer. New to reloading though so I’m pretty sure that’s where I screwed up and it feeds all factory rounds fine. I’ll attach some pics that may help although I am going to buy the recommended gauge. The side by side rounds are factory 62 grain with the green tip. Other round is mine. 77 gr smk with a 2.25 OAL. Other pics are the marker trick. A before and after. I can’t tell if the missing marker are from the trip up the feed ramps or not. Maybe you guys will see something. Again I really appreciate the help. This is frustrating to say the least.View attachment 7670610View attachment 7670611View attachment 7670612

You don't have enough port pressure or gas volume. H322 isn't really the burn rate powder you want to be running for the Mk12 spec Douglass bbl, port size & location.
IMR8208, H4895, VV N-135, VV-N-140, Varget (will be lower velocity but will cycle) or similar burn rate single base extruded will work much better. H322 is an awesome powder, just not for 77s in an 18", rifle gas system barrel.

 

[Daddyhorn]

You don't have enough port pressure or gas volume. H322 isn't really the burn rate powder you want to be running for the Mk12 spec Douglass bbl, port size & location.
IMR8208, H4895, VV N-135, VV-N-140, Varget (will be lower velocity but will cycle) or similar burn rate single base extruded will work much better. H322 is an awesome powder, just not for 77s in an 18", rifle gas system barrel.

Copy that. It was all I could get at the time. Slim pickings at the time. So if I have the bolt locked to the rear and insert a mag and hit the bolt release i get the issue. So not only when firing. It extracts easily when firing. Ha.

 

[FCS]

If you have access to Benchmark (bit fast in burn rate but is slightly slower than H322), IMR8208, VV-N135, H4895, VV N-140, VV N-530, VV N-540, Varget, RL-15, Norma 203-B...you'll have more success.

Ball powders would be: AA2460, AA2520, RamShot Tac, Hodgdon BL-C(2), CFE-223 or similar burn rate ball powder.

 

[FCS]

Copy that. It was all I could get at the time. Slim pickings at the time. So if I have the bolt locked to the rear and insert a mag and hit the bolt release i get the issue. So not only when firing. It extracts easily when firing. Ha.

You don't want to lock back, hit bolt release to chamber round as that is a enough energy to shove an case into chamber and "finish" sizing brass.
We are troubleshooting and testing in lieu of having tools that can tell you how much you are bumping shoulder.

I suspect there are two issues: 1 - Brass isn't sized enough and is making contact at shoulder when BCG strips, chambers round. 2 - Burn rate of powder isn't matched well to the gas port location, gas port size.

Running suppressed (Ops Inc 12th model or AEM5) or a different can?

Yes, powder selection is pretty limited so I fully understand those limitations

 

[Daddyhorn]

I’ll look for those when I go shopping for a comparator tomorrow. Ha

 

[Daddyhorn]

You don't want to lock back, hit bolt release to chamber round as that is a enough energy to shove an case into chamber and "finish" sizing brass.
We are troubleshooting and testing in lieu of having tools that can tell you how much you are bumping shoulder.

I used the bolt release at the range. That’s when I discovered the problem. Haven’t been using bolt release to trouble shoot. Was stating that to show that with all of that energy from the bolt release it still would not chamber from a magazine when I discovered the issue.

 

[nn8734]

I’ll look for those when I go shopping for a comparator tomorrow. Ha

AA2520 is solid at 24.9-25.3g in those barrels paired with LC brass (frankly most any brass will do). 2750ish in a 16” barrel, No pressure.

 

[FCS]

I used the bolt release at the range. That’s when I discovered the problem. Haven’t been using bolt release to trouble shoot. Was stating that to show that with all of that energy from the bolt release it still would not chamber from a magazine when I discovered the issue.

Ahhhh, I see.
My bad.

 

[alamo5000]

I built the rifle. Douglas SPR barrel from pri. PRI adjustable gas block. Rifle length gas system and buffer. Loaded with 21.8 grains of H322. I’m an active duty marksmanship instructor and the barrel was installed by a spec war armorer. New to reloading though so I’m pretty sure that’s where I screwed up and it feeds all factory rounds fine. I’ll attach some pics that may help although I am going to buy the recommended gauge. The side by side rounds are factory 62 grain with the green tip. Other round is mine. 77 gr smk with a 2.25 OAL. Other pics are the marker trick. A before and after. I can’t tell if the missing marker are from the trip up the feed ramps or not. Maybe you guys will see something. Again I really appreciate the help. This is frustrating to say the least.View attachment 7670610View attachment 7670611View attachment 7670612

Please retake the photos of the markered round. The second one is telling us something but the first one has so much glare I can't see it very well. Take it where there isn't some crazy reflection on there.

 

[FLIGHT762]

Screw your F/L die in another 1/12th of a turn, not too much. F/L size a couple of cases. Make a dummy round with no powder or primer and seat a bullet. See if that now chambers and extracts easily.

It probably will.

 

[alamo5000]

View attachment 7670612

What kind of die are you using?

First thing I would take the die out and clean it well. Secondly I think you haven't set your sizing die up correctly. Based on the second picture look at where the contact points are. At least one major point is right on the shoulder of the case, which tells me you are not sizing the case enough, which tells me that your die is not screwed down enough.

 

[alamo5000]

Screw your F/L die in another 1/12th of a turn, not too much. F/L size a couple of cases. Make a dummy round with no powder or primer and seat a bullet. See if that now chambers and extracts easily.

It probably will.

Agreed.

Maybe need a little bit more than 1/12th maybe, but let him experiment and see.

Definitely a sizing issue brought on from the die not being set up correctly.

 

[mtrmn]

If you just go ahead and screw the sizer down until it contacts the shell holder and load a couple rounds with it just to see if they'll chamber you can save a lot of time troubleshooting the sizing issue. Be sure and crimp exactly as before, only changing one thing at a time for troubleshooting. With your RCBS small base die set this will insure that you have sized the test rounds "enough." If they chamber properly then that's your issue. You can then back the die off -if you want to- until you get the setting you're happy with.

If you still have trouble then load a couple rounds without the crimp. And I HIGHLY recommend that if you crimp, do it in a seperate operation with a Lee factory crimp or a taper crimp die. With your roll crimp, each round has to be exactly the same length and the same neck thickness or some rounds will be over-crimped and some maybe not at all. The factory crimp and taper crimp won't damage the round when over-crimped.

When I load bulk ammo I set the die to contact the shell holder and cam over because I don't know which gun will be firing that ammo and it absolutely MUST fit and function above all else. IMO just "bumping the shoulder" is not for MY USE. I understand everybody else on here are shooting for the 'nth degree of accuracy but as you have experienced if the ammo won't function it's useless.

 

[Daddyhorn]

Please retake the photos of the markered round. The second one is telling us something but the first one has so much glare I can't see it very well. Take it where there isn't some crazy reflection on there.

Three different sides

 

[alamo5000]

Three different sides

Perfect. These pictures clearly show that you are not sizing the brass enough. Follow the post(s) about screwing your die in further and confirming as you go.

 

[Daddyhorn]

Perfect. These pictures clearly show that you are not sizing the brass enough. Follow the post(s) about screwing your die in further and confirming as you go.

Cool. I’ll try that as soon as I get home and keep you guys posted

 

[Daddyhorn]

We have success!!!! Picked up this Wilson gauge at the shop today. There was the tiniest of differences between my rounds and factory rounds but I guess it was enough. Cycles flawlessly now. Truly appreciate the help everyone. Thank you for being patient too. Most forums can get snotty. Thank you again