Bren uncommanded automatic fire event.

[5 Commando]

Hello all. I am a relatively new owner of a Bren MK3, 1956 vintage. She's in nice nick, with a good barrel and internally sound components. Today on the range it was cold, -10 C, and I was trying out some new ammo, (PPU 174gr), firing 5 round groups. On the second mag of the day - with the change lever in R - she fired an uncommanded burst of three rounds. I was somewhat startled, with the third round definitely ruining that group. I did an IA drill, and found the weapon would not fire subsequent rounds. The breech block would strip a round, but seemed to lack the power to fully chamber and allow the piston post to strike the primer. By way of background, I had inspected and prepped the weapon the night before and lubed all the sliding parts liberally with an automotive grease. At home the action felt silky smooth, but next day in the cold the action felt sluggish. Suspecting viscosity was the culprit, I stripped the Bren in situ and cleaned off the offending grease, lightly re-oiling with Balistol. The moving parts then seemed to move much more freely, and the rest of the day was uneventful, with the gun operating perfectly. Through all this I left the gas regulator at the smallest gas port position as it seemed adequate. I am still unsure however as to what caused her to fire that burst, and would value some advanced education here to prevent it happening again. Thanks, Mike.

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[Woodsy]

Your problem was caused by the grease you applied to the parts. This causes the moving parts to move slowly and the bolt travel was just enough to strip a round from the mag but not far enough to engage the sear. Grease on the sear could also make it slow to rise in cold conditions. Never use grease on a firearm, just light oil, and even then only sparingly. Grease and excess oil can cause problems in cold weather, and also act as a magnet for dust and dirt which then creates a fine grinding paste.

[Brit plumber]

Sounds very much like it short stroked, I think woodsy has it right.

[mrclark303]

I've seen similar situation here in the UK, a few years back with the guy I was shooting alongside at the range, he had a .22 semi auto (AR type) can't remember the make. Very cold day, grease and SV ammo .. he pulled the trigger and to his utter shock blasted the target with a full auto mag dump, half the mag gone before FTF.

Myself and the Range officer checked his rifle and a combination of poor cleaning practice, grease and old eley club ammo were rapidly diganosed as the issue.

[#1oilman]

Years ago I purchased an AR 10 Explorer .22 cal survival rifle from SIR in Manitoba. The day I picked it up at the post office I went out to try it out and to my suprise it fired full auto right out of the box and not just once, 2 out of 3 times it would empty the mag.

[ActionYobbo]

Like you I had problems in the past with my Mk2 when I took to it with bearing grease and gave all the working parts a light coating scrubbed on with a tooth brush. Took it out to the range when it was 22f or -6c and it was sluggish and would not run. I pulled it down and cleaned the grease off and it ran good. I now use engine oil for lube now and keep an oil can and brush in my range kit and a spray can of WD40.

[browningautorifle]

it fired full auto right out of the box and not just once, 2 out of 3 times it would empty the mag.

That would be very cool for an AR7...a different type of survival rifle... (I don't think you mean an AR10 explorer...) It went into the buttstock, right?

[5 Commando]

Thanks for the feedback guys. It does seem obvious in hindsight that the congealing grease caused the malfunction. This limited the piston assembly travel preventing the sear from engaging the bent, resulting in the burst of fire. I am surprised that the design allows rounds to be stripped from the magazine without sear engagement. I'm at the range today in more temperate conditions, -3C, and have also opened up the gas port one position. I'm workiing up some hand loads using Swiss Reload RS50 and PZN 174gr and at the lower end of the charge ladder I want to make sure the weapon cycles correctly. I will update with my lessons learned.

---------- Post added at 09:42 AM ---------- Previous post was at 09:39 AM ----------

Your problem was caused by the grease you applied to the parts. This causes the moving parts to move slowly and the bolt travel was just enough to strip a round from the mag but not far enough to engage the sear. Grease on the sear could also make it slow to rise in cold conditions. Never use grease on a firearm, just light oil, and even then only sparingly. Grease and excess oil can cause problems in cold weather, and also act as a magnet for dust and dirt which then creates a fine grinding paste.

Noted. Your analysis seems sound Woodsy. What is the graphite grease that I read about in the various MOD pamphlets? Is it available today or has it been superseded by better products?

[Woodsy]

Noted. Your analysis seems sound Woodsy. What is the graphite grease that I read about in the various MOD pamphlets? Is it available today or has it been superseded by better products?

Some light graphite greases are useful in sliding surfaces subject to high pressure, but I have always found a light oil to be perfectly reliable. Bren guns run very well on light oil applied sparingly (they even run well when quite dry).

[tankhunter]

The SECOND smallest hole in the gas plug. is THE standard position of usage, when in British Military usage. & always has been.