On an 870 if you have rearward pressure on the forend before the hammer falls you cannot continue pulling because the action bar lock is still engaged. The action bar lock will release from the jolt of a round firing (but not always with lite loads) or if you release the rearward tension on the forend.

Try it dry firing.

I have seen 870s that have a mod done that releases the action bar lock the moment the hammer drops. This allows you to pull through. They feel like they cycle much faster because timing is not so critical. Winchesters will allow for this.

My question is how is the action bar lock tuned to do this? I know shops like Tank's offer this service.

This isn't a slam fire mod like Ithacas or anything.

I would assume they cut the angle on the action bar and the action bar lock to a steeper angle and polish the interface.

HOWEVER, since I've never seen a Tank gun with this mod, I can't say for sure that's what they do, and I certainly wouldn't try it without being 100% SURE that's what they do.

In use, the fast opening of an 870 is more a matter of a well broken in and smooth gun, and extensive practice.

I've seen experienced 870 users who's guns seemed to open themselves the instant they were fired.
They'd just practiced and shot so much, they had the operation down pat.

The "trick" is, to put just the right amount of back pressure on the forearm, so when the round fires, the action bar lock releases and the forearm starts back, but not so much that the lock can't slide down the face of the action bar.

A year ago I've made such mode on my 870 just as it is depicted above: cut action bar angle and polished the interface. The problem appeared after that - sometimes new round fails to exit the magazine when action is already pulled back. The problem appears by chance during speed shooting only, but it is. I'm planning to purchaise new action block and new action bar to fix the problem in nearest time, but anyway - be carefull.

Exactly as stated above- the primary pressure of the hold needs to be with hand on the pistol grip. you also pull the weapon into the shoulder with this hand.
The hand on the forend should be holding firmly but not exerting rearward pressure.
By applying heavy rearward pressure all you are really doing is destroying the weapon.
This feature of the 870 is a distinct ADVANTAGE when shooting slugs accurately, because if you hold it in the above manner, basically nothing slides or moves after you press the trigger and the weapon discharges.

Quoted: With the 870, rearward pressure is applied mostly with the hand on the pistol grip while the hand on the fore end operates the action...one more reason to actually get out and shoot the shotgun more. John Satterwhite could routinely hand throw and break 7 clay birds before they hit the ground with his 870...I'd say that was pretty fast operation!

+1

Maybe you just don't like the pump shotgun concept.  Instead of focusing on rapid fire rate, why not give it credit for what it does best?  Takes a pounding and keeps functioning.  If you are so worried about how fast you can cycle an 870, why not just get a semi-auto?  

WhiteFox

"Amongst the blind, a one-eyed man is king."

Quoted: Maybe you just don't like the pump shotgun concept.  Instead of focusing on rapid fire rate, why not give it credit for what it does best?  Takes a pounding and keeps functioning.  If you are so worried about how fast you can cycle an 870, why not just get a semi-auto?   WhiteFox "Amongst the blind, a one-eyed man is king."

Because he wants the best of both.

-The durability and reliability of the 870.
-Fast operation.

"The best of both" is simply a display of ignorance and inexperience.  A person must choose his tools wisely to fit the need, and then become skilled at using them.  Sitting around fantasizing about some secret modification or other is a waste of time.  You can't buy tactics.  

WhiteFox  

Speed and the shotgun...

With a semiautomatic, how fast can you shoot?  With a semi-automatic, how fast can you shoot and hit your target?  There is a big distinction here.  If you actually intend to hit your target, you have to recover from recoil and realign your sight(s).  The heavier your load, the longer this takes.  With light target loads, you can likely light off two or three well placed shots nearly as fast as you can pull the trigger.  With slugs or buckshot, you've got between .5 and 1 second worth of time to come back on to target.  That is the time when the pump gun guys can cycle the action and be back on target just about as fast as the semi-auto guys.  You don't need to modify the gun to accomplish this either.  It just takes practice.

Speaking of practice, besides dry-practice (which is VERY important), try shooting sporting clays with your pump gun.  Don't go out there in full tactical gear, but get yourself a 28" barrel and head out to the local range.  If you really dedicate yourself to this, you'll find out just how fast you can be.

i purchased an 870 a couple days ago (NIB) and it allows me to do what i think you're saying.

i can have a round chambered and apply rearward pressure on the foregrip.  the grip is locked.  as soon as i pull the trigger, it allows me to pull the grip rear and begin cycling a new round.  this is on an unmodified shotty...

if i'm pulling back on the forearm w/ all my might, though, it remains locked.  normal pressure allows it to cycle w/ ease.