the shotgun or the shells?

[PolarPanda]

Hi all.
So, I recently had my 870 duracoated. After the coating, I gave it a thorough cleaning and lube job, then took it to the ranger for the first time in 8 years. I noticed right away, when firing slugs through the 20" rifled barrel, the slide was very very stiff... to the point of needing both hands to pull it back. I promptly stopped shooting, unloaded, took it home, dissassembled, cleaned thoroughly again, inspected, and went back to the range. On the second trip, the slide was noticably easier, but still dificult to cycle. I picked up a few of the hulls and brought them home. Upon comparing fired to unfired(but chambered) hulls, I noticed the fired hulls were marred up on the brass. Putting a caliper to them relieled that the brass part of the fired hulls were anywhere from .3mm to .6mm larger in diameter then the unfired hulls. The gap between the plastic and the brass was fully wide enough for me to stick my thumbnail down in. You can look and see that the thinkness of the brass is thinner on the slugs then on any of the other shells I have.
Now, I know the shotgun did nothing but sit in a gunsafe for 8 years while I was off playing Army games. I dont see how that could/would have an effect on anything. The 'smith that did the coating has a very good reputation, and once again, I dont see how that could/would have an effect on anything. With what I have listed about what I have found with the slugs, I am very inclined to believe that that is the problem, but am not completely sold on anything at this point.
Does anyone see anything wrong with the logic that the problem is with the rounds and not the shotgun?

BTW, Federal slugs.

TIA

 

[Oldnamvet]

Some questions:
Is the slide difficult to operate when there is not a fired shell in there? Can you give it a few rapid racks with no problem?
Did you give the chamber a real good scrubbing? Cleaning rod, brass brush, wisp of steel wool, and portable drill to remove any and all fouling there.
Did you try any other shells that had previously run smoothly?

 

[PolarPanda]

The slide opperates beautifully, whether unloaded and just working it, or just cycleing unfired shells through the action. For lack of a less sissy term, the chamber is quite pretty. No pitting, fouling, or any contaminates. 2.75" 00 buck and 3.25" 8 shot game loads cycle through just fine(the Federals are also 2.75"). Wanted to buy different slugs and try them, but the shop did not have any. My plan is to try and come up with several different brands and types of slugs today and see if they are any better.

 

[buck460XVR]

iffin it happens with spent rounds, it sounds like you may need to polish your chamber....happens quite often with parkerized shotguns.

 

[kirbythegunsmith]

Details

There would be the possibility of a chamber polish helping, if roughness was the only problem present.
However, your noting of the shell brass size points to the culprit: oversize chamber. Every shell fired will have some expansion of the case, and is designed to be smaller than the chamber to allow free insertion. The expansion of firing pressure forces the close fit of the case to the chamber for a micro-second, and then the case will have a slight springback, unless the swell/stretch is beyond the expected and designed amount, with a margin for tolerance.
Your chamber is above the tolerance for the shells that stick, and technically so even for the shells that might not stick.
I rework many oversize chambers to have a proper fit for the shells, and have seen the oversize nature arise due to excessive polishing as well as chamber metal expansion. Corrosion removal is one of the reasons that polishing may go unintentionally overboard and also cause uneven sizing.
Some barrels have come from the factory with chambers oversize already, and most are probably due to reamer walking during chamber cutting, and occasionally from polishing to remove reamer marks.
The brass expansion of .3mm (about .012") would place almost all shells at 0.814" or larger, and the chamber would be some amount larger in at least some part of the circumference. The .6mm is so far overboard as to be a surprise that you could get them out at all.
I do hope that the refinisher did not polish your chamber in an attempt to remove residues or other condition from the finishing process. Many amateur polishing jobs will make the chamber larger more near the rim edge than the full length, but if there was some side wallowing, there could be a hollow that the brass could use for an expansion "toehold" to cause pumping resistance.

The attached picture is one example of swelling near the rim edge due to an oversized chamber (mainly near the chamber rim edge).

See my previous postings for more gunsmith insight.

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