AR15 and AR10 buffer spring compression question

[copterdrvr]

This might be alittle confusing but I know someone here can answer this probably stupid question....

 

I installed a POF .308 buffer tube on my FM-9 lower which is .75 inch longer than a standard carbine buffer tube as well as adding one quarter in the bottom of the tube and a Kyn-shot HD hydraulic buffer. I'm shooting a 100 grain bullet pretty slow and was looking for the lowest bounce from the gun.

 

Here is the question-I know that the bolt bottoming out against any part of the lower is disaster so with the lower separated from the upper, I installed an AR15 rifle spring on top of a quarter and installed the hydraulic buffer. I then installed the bolt and pushed it into the buffer tube. I could feel the hydraulic piston bottom out and with CONSIDERABLE PRESSURE could not make the key contact anything with a clearance of about .10 inch.  I then installed a 308 carbine spring, repeated the test and got the EXACT same result using my entire body weight to attempt to compress the spring farther.

 

WHY?

 

To add, I have a few thousand rounds through the rifle with no evidence of bolt contact using the AR15 rifle spring and my main question is why does the AR15 RIFLE spring and the AR10 carbine spring compress the same amount. They are both JP springs.

[copterdrvr]

And in case my question is not clear enough-why does the AR15 RIFLE spring compress the exact same distance as the AR10 CARBINE spring as it's a bit over an inch shorter. 

[copterdrvr]

Come on, there has to be a spring expert here!!!

[mpom]

No spring expert here, but my understanding is the spring can only be compressed till the buffer hits the closed end of the buffer tube (actual name is receiver extension).

So a shorter buffer allows for more spring compression than a longer buffer. Since you don't want the bolt carrier to go too deep into the tube, you match the buffer tube with the correct length buffer.  If you switch springs of different lengths but keep the buffer and tube same, why do you expect a change in how much of the spring gets compressed? If you replace your rifle buffer with a carbine buffer, then you will compress more spring, and likely damage something as more of the bolt carrier enters the buffer tube than should.

Mark

[copterdrvr]

Well, you're absolutely correct and my error was assuming that the rifle length spring would "bottom out" or reach compression BEFORE the hydraulic buffer bottomed out (because it's in a less than rifle length receiver extension) and my own observations obviously proved that is an incorrect assumption.

 

Thanks for the response!

[RaylanGivens]

On 10/8/2020 at 2:14 PM, copterdrvr said:

And in case my question is not clear enough-why does the AR15 RIFLE spring compress the exact same distance as the AR10 CARBINE spring as it's a bit over an inch shorter. 

 

Because the spring does not determine the overall travel of the bolt/buffer unless the coils bind.

 

Which KynShot buffer are you using?

 

[MemphisMechanic]

A coil spring should never be the element that determines a slide or bolt’s travel length. It can only do so by reaching full coil bind, which is less than ideal for the spring.

 

We use the 308 and/or rifle springs for more spring weight, not to reduce travel.

 

In an AR, the buffer is sandwiched between the bolt and the bottom of the buffer tube and it is what stops rearward travel. That’s exactly why people stack quarters in them - to reduce travel length since you cannot do so by swapping springs.

 

Edited October 13, 2020 by MemphisMechanic

[copterdrvr]

I'm using the Kynshot HD buffer and I now realize that neither of the springs are fully compressing so it's not an issue. Should have figured that out on my own!!!!

 

Edited October 13, 2020 by copterdrvr

[RaylanGivens]

On 10/13/2020 at 6:17 PM, copterdrvr said:

I'm using the Kynshot HD buffer and I now realize that neither of the springs are fully compressing so it's not an issue. Should have figured that out on my own!!!!

 

 

I assume you mean the KynShot rb5015hd buffer?

 

So you added a buffer tube that is .75" longer and then only added one quarter to make up the difference?

 

https://www.blitzkriegcomponents.com/ProductDetails.asp?ProductCode=R15015098

 

Try adding one of the KynShot buffer weights to the front of your 5015 buffer...  You will still need to add some more quarters even with the buffer weight...  Add quarters behind the spring to limit the bolt/buffer travel...  Just make sure the bolt travels far enough behind the cartridge to still grab the next round from the magazine.