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extracting a live round with the charging handle almost never worked ... Unfortunately, the problem still hasn't been fixed.

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I'm also having a mag problem, in that the internal mag binds up in the outer shell when removing or installing it. If any one can suggest a way to fix that, thanks in advance.
Hello again. I actually didn't really understand what the OP was referring to. Is it a failure of last-round-bolt-hold-open?

For the first issue you mentioned, it really does sound like your rifle isn't fully in battery when that unfired round is intended to be extracted.

Using Steelworx metal dummy rounds, I was unable to recreate your situation.
https://www.amazon.com/STEELWORX-Steel-Snap-Dummy-Rounds/dp/B07X1ZS5JG I realize these aren't "real" cartridges but at least trying these should highlight any anomaly, safely.

Powder residue buildup at about the 9-10 o'clock position of the breech face is not uncommon for me even after 150 or so rounds. Not saying this is the problem or even A problem, it's just something to look for since I can't imagine another cause that's obvious. I can imagine PLENTY of non-obvious ones!

Regarding the plastic-on-plastic drag on the magazine parts, I at least partially mitigated this expected scenario by using a very light silicone oil wipe of the inner section. Try also putting the magazine in the fridge for 20 minutes and see if it still has the issue. I suspect that wear of the plastic mating surfaces will diminish this tolerance issue over time.
 

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"To clarify, you are suggesting that the powder build up might be preventing the extractor from grabbing the lip of the unfired round?"

More specifically, if there's something preventing the bolt face from squarely coming to rest in battery .... and this is the situation you check with your snap cap. Take the upper off and manually move the bolt against a chambered snap cap, watching the action of the extractor.

"Which brings to an issue I need to address, which is how best to clean the M4s in terms of products and steps. A gunsmith mentioned that I might be running the firearm too dry. If you have any suggestions, I'd be very interested."

Since this is a simple piston machine, I go by the idea of removing impediments to the piston operation, removing overthinking and gun lore. ;-) Remove upper from rifle, then remove bolt. Go outdoors because this stuff is smelly. Point barrel up. Spray 'blaster' type cleaner (large spray can sold at Walmart for around $8) around in breech area, focusing on getting rid of debris buildup. Use rag tied with string, dragging through where the bolt travels to remove carbon and the remains of the factory paint overspray that wore off. While you have the can, spritz the face area of the bolt with the blaster and wipe off with the rag. A toothbrush is handy to make really certain the bolt face has no combusted debris around the firing pin.

Lubrication is a different story. On the bolt you will see the 'drag' marks where the bosses in the upper 'guide' it. Oil those with something light, like Hoppes or even Breakthrough CLP. Clean and let dry the extractor and firing pin channel using Eezox.
 

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To clarify, does this refer to Birchwood Casey Gun Scrubber?
There are several brands they carry, depending on stock, this is one of them and I don't find much difference between them. These are aggressive cleaning type chemicals and they quickly strip oils from protected surfaces. Obviously use volatiles like this wisely and with plenty of ventilation. Don't let these drip on stuff that could get damaged. Don't get these on optics.

If 22lr combustion debris gets crusty or stuck in a crevasse, using these can help:
https://www.amazon.com/Tipton-Firearm-Cleaning-Picks-Maintenance/dp/B08X4SDTHG
 
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