Blown Primer in Wilde Chamber (Update w/range results)

Originally Posted By: WebopperIf you jump to page 2, you'll see where he sent his back in and BM said the chamber and headspacing was too tight. They fixed it up for him, and it's all good.

http://www.ar15.com/forums/t_3_16/479900...html&page=1

Don't think I'll let anyone mess with a barrel that shoots like this one
wink.gif
, especially since it has never exhibited any signs of excess pressures w/various factory ammo or handloads. Only pressure signs experienced was w/M193 ball (excessive case head expansion on two cases and one blown primer out of 5 rounds fired).

This particular load is above max. in three different manuals, chronographs 3300 fps 15' from muzzle and, loaded to max. magazine length measures 2.264" COL. yet shows no signs of pressure in MY rifle.

34931632143_fb7e289976.jpg


I do appreciate the fact that your post caused me to check one parameter that I had not checked to date and think we may have struck on a possible contributing factor, if not the primary cause of the blown primer.

While checking the COL of my handloads, it occured to me to do the same on the M193. First thing I noticed was that the cannelure was not seated to same depth on the 5 cartridges selected at random.

Case base to ogive measured w/Stoney Point bullet comparator:
My 55 gr. NBT handloads: 2.8585-2.859

M193 Ball:
2.902
2.904
2.908
2.913
2.915
M193 variation of up to +/- .013 (which is obviously not conducive to good accuracy!)

Handload variation of +/- .0005

M193 base to ogive(longest)= 2.915
Handload base to ogive...... = 2.859
Less "bullet jump w/M193.. = 0.056"

That's a lot of difference in free travel, no wonder pressures running a great deal higher in the M193!

Regards,
hm
 
Originally Posted By: 6724the problem i had with my armalite ar10t was too large of a gas port. there was enough gas being applied to the bolt that it was opening before the pressure dropped enough. so when the bolt was unlocking, the pressure was still high enough to expand the case head. all ammo did this, some more than others.
my above statement is my THEORY of my situation. however, your situation may be similar. i would fire it with some different ammo and see what happens. if that blows primers or expands case heads, then i would suggest that it is the rifle.

Originally Posted By: hm1996 It has relatively low round count, somewhere in the neighborhood of 600-700 rounds through it. When I first purchased the rifle, I did shoot one box ea. of different factory loads down to and including Win. white box to establish a base line, all functioned perfectly and grouped 1 moa or less. It has since seen an exclusive diet of my handloads which also function flawlessly.

I think we have pretty well established that there is no fault with the rifle, just an ammo problem.

Regards,
hm
 
I've been thinking about this and just about decided to push a few of the M193 bullets back to same head to ogive measurement as my pet handloads (which are, as previously stated .056" shorter than M193, & loaded above max. loads listed, yet show no pressure signs in my rifle)and I'd bet a steak dinner the pressure signs go away.

If I decide to do so, will single load the rifle to avoid what I consider to be the major danger and that is a dislodged primer slamming the next live round as it chambers causing an out-of-battery slam-fire.

I'd bet the accuracy of this ammo improves a bit with a uniform bullet seating depth as well.

Regards,
hm
 
Originally Posted By: hm1996

The M193 ammo case base to ogive measured w/Stoney Point bullet comparator:
My 55 gr. NBT handloads: 2.8585-2.859

M193 Ball:
2.902
2.904
2.908
2.913
2.915
M193 variation of up to +/- .013 (which is obviously not conducive to good accuracy!)

I've been thinking about this and just about decided to push a few of the M193 bullets back to same head to ogive measurement as my pet handloads (which are, as previously stated .056" shorter than M193, & loaded above max. loads listed, yet show no pressure signs in my rifle)and I'd bet a steak dinner the pressure signs go away.

If I decide to do so, will single load the rifle to avoid what I consider to be the major danger and that is a dislodged primer slamming the next live round as it chambers causing an out-of-battery slam-fire.

I'd bet the accuracy of this ammo improves a bit with a uniform bullet seating depth as well.

Regards,
hm


Decided to tear down 10 rounds of the lot of M193 in question and found that powder charges (with an unknown, fine ball powder which resembles Ball C) varied +/- .5 grains (seems like a lot for such a smooth metering powder). Case weight variation was +/- .7 gr.

Checked bolt face to rifling on my rifle @ 2.940".


Decided to re-assemble, leaving powder charges as thrown at the factory, but pushing bullets to achieve 2.866" (both chamber and loaded rounds as measured w/stoney pt. comparator) giving .074" freebore (as opposed to 2.91 COL (average from factory) for an average freebore of .030").

It was 96* at range today. Fired all ten rounds w/no blown primers and entire group measured 1 9/16". Mirage was heavy and rest was a rolled up gillie jacket rear and rolled up hand towel on a brick front, so not too bad considering.

34899338754_6162881bd6_c.jpg


Here's some pictures of case heads; a couple had slight smear on head but no ejector imprint as with the longer rounds fired last week.

Expansion Ring Measurements for comparison:
M193 as loaded Shortened M193 Hot Handloaded WW cases
3.77 blew primer..3.72 (3)...............3.74 (3)
3.75 held primer..3.725 (2)..............3.742 (1)
..............................3.727 (3)..............3.743 (1)
..............................3.728 (1)..............3.75 (1)
..............................3.738 (1) No blown privers in WW cases

35571239882_dbe96d111b_c.jpg


35700104346_6c320366af_o.jpg


So there you have it, as one would expect, chamber/COL does matter; combination of soft brass didn't help IMHO.

Planning to set the rest of the bullets back and call it good.

Regards,
hm

 
Sure seems like some stellar critical thinking/deductive reasoning and/or variable tweaking/elimination of variability.

Sir, my hat is off.

-S
 
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