[Raven1st]

Maybe by a few FPS, but if your regulator is good I don't see any issue in running HPA to the cartridge.

It's also firing 3-6bbs at the same time on gas, it's not very consistent to begin with.

 

[SomewhatInsane]

15k rounds through my HPA'd M870 this summer with no problems. Running it at 115 PSI which gives me 340-342 FPS with a .2 gram bb.

Things I have done to improve performance/gameplay experience for me:
Re-fitted the HPA line. I ended up taking out my cobbled together line and used the exact same method as wingman. No more kinks, or leaks, at all.
New barrels. I now get a substantial increase in range on 6 shot mode.
Changed BB's. I went from running .25's to running .32's. On six shot mode the .32's are still moving fast enough for a long enough distance to hit people even in the furthest shots I can make at my field. They're also much louder. Which is extremely helpful for guys with plate carriers. I found that .25's would never even get noticed...
MOUNTED A BAYONET.
This was the single best thing I have done to this gun. I took a cold steel training knife, bolted it into a scope ring and put it on a rail adapter at the front end of the shotgun. Melee kills galore, looks wicked, and nobody complains that I shot them too close now.

The next thing I want to do is buy a sidesaddle shell holder. After disassembling the gun so many times I've found that the body pins are really quite loose. And since the shell holder has bolts that replace them I'll be able to kill two birds.

 

[Wingman703]

MOUNTED A BAYONET.

Need pics for... ah... research purposes.

 

[SomewhatInsane]

Need pics for... ah... research purposes.

No idea why they're upside down but... also don't care...
I've been averaging 7-10 kills with it per day of play.

 

[SomewhatInsane]

Some updates:
My original M870 Tac (circa 2014) has recently suffered from shearing off the actuator pin on the loading plates. This occurred at right around 40k rounds of hard use. All other mechanics of the gun work fine.
Will be fixed, once I decide whether to get the WiiTech aluminum loading plate set, or the SAT steel loading plate.

My local field had to close its old location and open anew in a new area, and as such my setup changed a bit. The ranges were shortened and overall area tightened up so I ditched the Aimpoint and bayonet (most days) in favor of the low profile irons, and a flashlight. Since we are indoors 100% of the time.
Still been running the gun at about 87-90PSI getting about ~290 FPS with a .25.
3-shot mode is king right now, because reloading is a hazard in fast paced CQB and I haven't gotten an M4 magazine adapter yet.
Early on when I started playing again my air-tank fill valve blew up, so I started playing with the M870tac with the Breacher grip on it. Fun setup and great for CQB.

My M870 Breacher is going strong, I bought it secondhand and have put about 7-10k rounds through it, since my full size broke down. And I've been pushing the same 90PSI for ~290 FPS with .25's.
Attachments/setup wise on this one all I've done is HPA the Angry Gun stock adapter, (with a super sh*tty LPAEG M4 stock...) simply as a method to fix the hole that was unintentionally put in the gas tube. And beyond that I have added an aluminum G&P top rail/shell holder.

The G&P shell holder isn't my favorite, tbh. I find that it holds the shells REALLY tight, and takes some force to get them out. This makes it kinda tricky to reload in stressful situations but I've gotten quite used to it and broken it in a smidge.

I also snagged an M870 (fullsized) on HopUp that had the barrel cut down, and needed a bit of work. Including a finishing of the cut down barrel, fixing the end cap threads (had been smashed in) replacing some internals that had been left out for some reason? And since the previous owner had a Magpul real-steel M870 handguard for it that he threw in (He couldn't get it to fit, obvi...) I spent about 6 hours filing/rotary tooling material out until it fit and functioned well. For anyone wondering, this fore-end is actually worth the time and work. It's comfortable, has the ability to add railspace and an AFG, and it looks great.
The gun still needs some TLC, attention and a bit of fit and finish but otherwise not bad for the $100 I paid for the gun, angry gun stock, MOE gandguard, and OEM parts...

I also picked up a KSG from an extremely generous gentleman on hopup who not only sold me the gun dirt cheap, but also paid to ship/insure it.
The current KSG setup is the gun, 4 gas tanks, the DTD mag adapter, and an AFG.
Now, I'm gonna be honest, I didn't like this thing the first round I used it in. Or for the whole first day. I was using a super crappy M4 highcap that I took off a rental that was lying around, and that had feeding issues. But more importantly, I couldn't aim the gun for crap. The bore axis was SUPER janky for me after having played with the M870 for almost 5 years straight. That made shooting around corners, over bunkers and through holes really tricky. On day two however, I decided to run just shells, and added the AFG, and it really excelled. I was starting to get used to the bore-axis, the AFG helped with the super mediocre fore-end, and the shells ensured 100% reliable feeding. Reloads are tricky though.

So, the current list for what I'd like to/need to do as far as these beasts go, includes.

1) Replacing the loading arm on my original (and still favorite to use) M870 tactical. I first need to decide which loading plate. Recommendations would be helpful.

2) Choose which one and acquire an M4 magazine adapter for my 870. I'm leaning towards the Salamander, because it's injection molded and will hold up better to repeated use.

3) Fit and finish on my $100 870 project gun. Need to find an endcap for the magazine tube, and fix those threads, as well as re-install a shell door and fake bolt/bolt cover, and fix the end of the barrel.

4) HPA my breacher grip. Probs by going through the gas tank. Just seems like the overall best option, I guess.

5) HPA the KSG. I'm on the fence about keeping this, just because I'm a little less familiar with teaching on it and maintenance/upkeep. It just makes me nervous to play with.
On the other hand I know if I HPA it I'll use the living hell out of it and have a blast doing it.

6) I'd like to do a second Magpul build, with the Angry Gun or preferably the G&P stock adapter. If I could find one... (if anyone has one please sell it to me! PM me!) I have a line on another M870 Tac, that needs some help so I could get that cheap, and then do an FDE build, with the FDE MOE fore-end and collapsible stock. Would probably add an extended shell tube and keep the full length barrel, as well. And that would be a sort of fun last variation to add to the pile.

Gameplay wise, I carry 6 shells on the gun, in most cases, and 12 in a pouch. I'm considering stepping that up a fair bit by another 12 shell pouch because I'm starting to get into 6 shot mode, and I have tons of shells.
I also carry almost nothing on me. My loadout consists of jeans, a T-shirt on hot days, or a long sleeved on cooler days, and a surplus U.S web belt with my shell pouch, a zip up pouch for spent shells, and a universal holster for my FNX45. When I'm HPA I use a $13 backpack off amazon to hold the tank. So I go super lightweight and mobile, stay low when I can, and like to push/flank/push and then go back, and repeat as necessary/able.

A lot of people talk about wanting to get an M870 or a scattergun or a KSG or whatever, but seem to be put off by what they perceive as a lack of firepower compared to AEG's but to be quite honest, after almost five years of playing with nothing but shotguns (pistol as a sidearm, used maybe twice a week) the lack of firepower narrows to almost nothing. I can empty a 30rd shell on three shot just as fast as most people can shoot 30 rounds out of an AEG on semi auto, and most low-mid-entry level stock AEG's on full auto. Furthermore, the ranges are virtually equal, as far as most practical CQB engagements. And they're WAY more fun to use.

 

[Raven1st]

I really need to get on setting my M870 up for a remote gas tank. The stock ones have been starting to leak after a little more than a year of light use and maintenance.

I find the 870's to be better on firepower than an AEG in a CQB arena.
The local CQB field has Semi-auto Sunday games, so everything (even LMG's) are only allowed on the field with SA firing.
Multi-shot shottys count as semi-auto guns.
6 shot mode FTW.

 

[Cheesydebris]

Since ArniesAirsoft is gone, I have no outlet for 870 questions. I have a breacher 870 that now only fires 3bbs on both 3/6 settings. Can anyone point me in the right direction for troubleshooting? The spring just below the toggle switch is still on the nub, that's as far as I've gotten looking at the replica.