870 Capacity - why only 6+1?

[Akurat]

Riddle me this:

In my 870 HD, the extension tube is crimped a few inches from the end (NOT refering to the dimples), preventing the follower from going back any further and allowing 6 cartridges in the tube. Aftermarket extensions for the 870 are also only "6+1".

Mossberg, Winchester, and Benelli shotguns are all capable of 7+1 capacities in their 18.5" models. These guns have exactly the same length tubes, but fit one more round. Barring the obvious crimp in the factory Remington extension, what prevents 870's from achieving the same 7+1 capacity with an aftermarket extension(ending flush with the 18.5" bbl) ? Are the mag springs just shorter? That's the only explanation I can think of, because it just doesen't add up.

 

[mrk]

i believe the mossberg 18.5 only allows 5+1 and doesnt allow a mag extension either

 

[Akurat]

The 5+1 model is irrelevant to the question.

 

[Remington788]

I think the 7+1 capacity is due to 20 inch barrels not 18.5. I have a 11-87 with 21 inch barrel that has a 7+1 extension on it that is 1 inch shorter than the barrel and a Ithaca police deerslayer with 20 inch barrel that has a 7+1 capacity. The 870 we have here at work only has an 18 inch barrel and can only hold 6+1, the crimp on the end is for the support band that goes around the barrel and extension tube. Besides, the spring in the tube can only compress so much.

 

[Akurat]

True, but if that crimp below the clamp were not there to obstruct the follower, I believe that one more shell would still fit.

Example: Winchester Defender - 18" bbl - 8 shot
http://www.winchesterguns.com/prodinfo/catalog/detail.asp?cat_id=512&type_id=209&cat=012C

Also, line up seven 2-3/4 inch shells along the 870 tube, or just measure it out.

 

[dfariswheel]

The crimp serves two purposes on the factory extension.

First, it interlocks with the clamp screw and prevents the clamp from moving under recoil.

The 20" barreled guns were originally available with a bayonet lock on the clamp.
The crimp also served to position the clamp in the right spot so the bayonet could latch onto the mount.
When the original factory bayonet system proved to be too weak for actual use, the military developed a different bayonet mounting system.

Second, the crimp prevents attempting to forcibly shove an extra shell in and causing the magazine spring to be over-compressed, by providing a compression area for the spring.
In extreme cases, this can cause the magazine spring to become entangled, jamming the gun.

Non-factory guns without the crimp depend on the user to not attempt to force another shell in.

So, the crimp is to position and lock in place the barrel clamp, and prevents over compressing the magazine spring.

 

[Akurat]

That's exactly the answer I was looking for. Explains it perfectly.

Thanks, dfariswheel.

__________

Because I like to tinker - I'm thinking of slightly shortening an 870 mag-spring in order to lessen the compression just enough to cram one more round in (using an aftermarket tube without the crimp). I figure that if it can be done in another 18" gun, it can be done in an 870 as well. I know that shortening the spring may render it relatively weak when there is only one shell remaining in the tube, so that will be the main concern *if* I can get 7 in successfully.

Anyone tried something like this?

 

[1911JMB]

It can't be good for the spring, but I can fit 8 shells in my 870 when the "+2" extender is installed.

 

[mrmeval]

Why not get the 10 shot drum magazine conversion kit?
Then you can get it converted to a shorty.

 

[357wheelgunner]

Because Remington is all about reliability. Even if the COULD stuff one more round in there, they would not do it if there was a chance of overcompression of the spring or other malfunctions. You can't compare the 870's capacity to the capacity of less reliable weapons. Apples and oranges you know.

My 20" 870s hold 7+1, but sit loaded with 4 rounds in the magazine to help the spring last longer. 5 more rounds sit in the buttcuff. If you can't solve a problem with 8 rounds of buckshot/slug you are in over your head and it is time to either grab the battle rifle and the body armor, or call the national guard.....

 

[Akurat]

I don't know when the Winchester 1300, Mossberg 500, or Benelli Nova series became less than reliable options, but we'll leave that to another thread..

It's not as if we're going to cram 8 shells in and never test the gun for reliability. Also, spring strength is more about how many times a spring is compressed, uncompressed, and recompressed--rather than how long it is left compressed. There are other threads on that matter as well..

On the issue of capacity, how many rounds is good enough? "If you can't solve the problem with X rounds, then [insert exaggerated solution here]" arguments sound nice, but so do "one more shot can save your life" arguments. In a self-defense situation, I don't expect my target to line up and be still for me. I don't expect to be able to get into my usual firing position or to completely maintain my sense of calm. Hence, I don't expect -- realistically -- to hit my target 100% of the time. You just never know.