WS6

This is caused by a weak magazine spring. This is an inertia operated gun and so I know you understand the principal of inertia which makes explaining this easier. What is happening with the bolt/inertia spring, ALSO happens in the magazine tube. If recoil is strong enough, the shotgun shell in the tube wants to stay in place while the gun moves backwards (inertia). If the spring is weak, it is allowed to do so to whatever extent. My M4S90 would do this (bolt lock-back, or appear to) when I ran a full-length tube with the OEM spring and shot heavy buckshot loads. Replace the magazine spring and go from there. You may need a different spring as well, but the mag-tube spring is the most common culprit when someone says "Last shell/heavy loads/etc."

On a serious note. The Rig should be okay. Personally I use Never Sieze. And I give the tube a check between stages too. RIG+P is thick at room temperature, but if you get it hot it quickly liquifies. I strongly recommend something that does not evaporate and handles heat/pressure well. RIG handles pressure nicely, but I prefer something with a metallic base that will NOT! evaporate/burn-off, etc.

I used to have this problem. I bought a Benelli M4 (my first of 3), very excited. I lubed it well, tightened the choke down with the factory wrench, and went to town! Every box of shells, it would loosen. I would dutifully tq it down again. Again and again this was repeated. So I posted on the internet. "Use grease" came the response. So I did. No help there. I just now had something that wouldn't wash out of clothes (Birchwood Casey metallic-base choke-tube lube, which I still use and love, btw.). Anyways, what finally solved the issue was a Briley Speed Wrench. I now TQ it down good and snug, and last outing I shot 15-20 rounds of buckshot, 5 rounds of steel DDupleks slugs, 15 Slugger slugs, and 25 rounds of birdshot, (some rapid-fire 9-shot mag-dumps included). When I got home, the tube was nice and snug. My OEM wrenches just collect dust now, and I think those cheap-looking "quarter with guts" style wrenches are a total joke. Bit the bullet and buy the Briley speed wrench, put some juevos into tightening it (don't get rediculous, just "manly" tight is good). Use a good choke-tube lube as well. If carbon gets under the tube, it won't be able to really solidify with that grease in there. Also, the grease will prevent it from getting stuck for any other reason. Non-evaporating up to about 2,000*F I think it says on the tube. I leave my choke-tube in after spraying it and the threads with CLP and then greasing the threads thoroughly. Just what worked for me, your situation may vary of course!

I don't mean to resurect a thread. I am new to this forum and have not seen this one answered. Can you be more specific? Does it do this on ALL shells, or the first ones, or last ones in the mag? Please give me more info and I can probably pin-point your problem. I have owned 3 M4's and played with mags, springs, followers, etc. etc. and dealt with a few quirks myself.

I know for rifles this is a big deal, especially AR's using XM193 or magnum rifles loaded within an inch of their life, incase a primer ruptures and damages the pin, but is a primer rupture common with a 12-gauge at that low of an operating pressure? My firing-pin looks very similar to the M16 firing-pin and doesn't look like anything short of a primer rupturing and cutting-torch-ing the tip would hurt it. Am I not thinking about something? Trying to stock up on spare parts myself but didn't think the FP was easy to damage.

This is kindof worrysome as it seems "over-gassed" with the low-power piston. If the "heavy" piston will fire bird-shot, then firing anything but low-recoil loads (beanbags, etc.?) with the light piston would be asking to cause damage similar to the broken pistons suffered by the early Benelli M4's that used the 4-port system, no? I don't own an SLP, but this is just a thought. Feel free to point out my error.

Magazine capacity is a wash, and weight is almost identical. I am more interested in which guns seem to run better, have less problems, and which one seems to pattern the best, as a rule? I have an M4 and am contemplating adding an FN SLP (18") to my collection, but only if it offers something my M4 doesn't already. My M4 doesn't get real dirty and is very easy to clean when it does. All the moving parts are metal, there are no rubber "O" rings, etc. except as seals or retainers, they do not appear to be critical parts. Is the FN SLP as robust? Any advantages to the SLP? So far what I can determine: M4=9 rounds, SLP=8 rounds (both capacities factorying in "Ghost loading", as I was told the SLP can do this too) SLP= 7.3#, My M4= 7.5# SLP= Chrome lined bore, M4=chrome lined bore and bolt-head M4=easy take-down of every moving part, including the bolt, in minutes. SLP=??? Both have choke-tubes. M4 is dual pistol, SLP only has 1. M4 auto-regulates, SLP needs 2 pistons. My M4 has NEVER! Jammed using anything from 1145fps 2.75 dram bird-shot to 3" Magnum buckshot as long as I do my part and hold it tight when firing the light stuff. Trigger-pull feels like a mil-spec AR. Have not gauged it, but would guess it to be 5-7# and crisp enough. Very similar to the mil-spec AR triggers I have. I can sometimes "outrun" my M4 as the disconnector will allow the trigger to be pulled during the last 1.5" of bolt-travel, leaving you with a dead gun and a live round in the chamber. This only happens when I fire it holding it loosely or from the hip using bird-shot, all of which has the effect of slowing bolt-travel. Thoughts? Opinions?

Purchase the spring from www.carriercomp.com When I added a full-length tube to my M4, I went through: Surecycle Wolff Generic 40" spring for a Remington? Benelli M1S90 Military spring. NONE OF THEM would function reliably and still allow 7 rounds. In fact, the only one that worked was the M1S90 spring, and it was quite obvious by the poor fit in the follower and the lack of being able to load a 7th round that it was masquerading as something it wasn't. The carrier comp spring is designed from the ground up to function in 7-shot Benelli M4's. Mine has been flawless. The spring was spec'ed by Kip and made by Wolff as I understand it. It is NOT! the same rate as Wolff's generic 40" or whatever springs. It is much better.