Strange but VERY welcome AR-15 accuracy increase!

J

JR Hartman

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So, I wanted a spring for my Remington, and I needed to buy something else to meet the minimum Canadian shipping from Wolff.

I poke around, and lo and behold, I add new AR-15 mainspring to the cart.

It shows up, I pop out the old one and note that its not the same length as my current one. The new one is like 2-3 inches longer and a bit more firm feeling.

But, Wolff knows what they are doing so, I put it in and reassemble the rifle and head out to the range just to test function.

I blaze away with a few mags full of bulk 55 grain ammo and its just fine. Zero problems. A bit less "slam" feeling on firing which is welcome, and the pile of brass is both closer to me and in a smaller pile. Also welcome. :)

Next, I try some of my "weak" loads of 21grains of TAC under a bulk 55 grain pill. Still no malfs of any sort. pile is a bit closer.

Then I figure, well, there is a target out there that is still clean.

I slap the rifle on the front rest but the rear rest is in the bag so I just hold it as best I can. I just get a sight picture and snap off a few rounds. When I see the target I am very surprized! So I try it again. Second time I try a bit harder, but still fairly rapid fire. Probably about 1 every second. No proper breathing, no rear bag, no "zen" stuff, just shooting. Now, the 10 rounds are basically all in the 10x center of the target. probably dropped 2-3 in the next ring. You know, the Canada Ammo red targets. Yes its only at 50m, but before the groups would easily have been 2-3x this size.

I fired about 3 more mags, same results. All bulk 55 grn FMJ ammo or the 21grains of TAC loads, using multiple fired random range brass including all the regular cheap brands of brass.

Any ideas? I am not complaining by any means, but am just curious as to why.

My thoughts are that the stronger spring is holding it in battery a bit longer changing pressure or harmonics or something like that, but I don't know.
 
JR Hartman said:
It shows up, I pop out the old one and note that its not the same length as my current one. The new one is like 2-3 inches longer and a bit more firm feeling.
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2-3 inches longer? Do you have a carbine receiver extension? If you do, you have just placed a rifle buffer spring (around 12.5") in a carbine that takes ~10.7" buffer springs. This is not good.
 
hilarious! :)

Yes, mine is carbine length and I suspect its a rifle length spring, but I am not changing it out unless someone can tell me specifically why its a bad thing.

Seriously, the groups are cut in half or better. Mostly better, with zero malfunctions. And the baseball size groups before were shot using proper breathing, rear bag etc. The loonie sized groups were shot like stated above.

The odd thing is that the rifle used to shoot like this when new, but its slowly been getting crappier and crappier, to the point that it was pre the new spring swap. I figured I had just started to erode the throat or something because its approaching(or has exceeded 5000 rounds).

The groups immediately tightened up, and the results were repeatable for sure. I probably shot 150 rounds thru it after the spring swap and it was all tighter groups.
 
Ok so you are observing 1 behavior thats better, accuracy.

But what about the behaviours you are not observing?

I sure wouldnt be putting a rifle length spring in a carbine tube and carbine buffer weight. The pressure in a rifle length system is lower than a carbine, therfore you are compensating lower force so having a longer spring is fine, thats why the action is so smooth on a rifle length. But you don't have a rifle gas, you have a carbine gas.

So imagine your bolt cycling back at a force that is compensated by the wrong lower resistance, there is still substantial force left at the end of the system that needs to be dissipated, which it does by transfering it rather forcefully onto the end of your tube, so your bolt, tube, buffer weight, spring, will suffer wear that it was not intended to suffer.

If the accuracy gains are worth it, and you never intend to sell it later on without disclaiming that fact, its your rifle...
 
mikethebike said:
Ok so you are observing 1 behavior thats better, accuracy.

But what about the behaviours you are not observing?

I sure wouldnt be putting a rifle length spring in a carbine tube and carbine buffer weight. The pressure in a rifle length system is lower than a carbine, therfore you are compensating lower force so having a longer spring is fine, thats why the action is so smooth on a rifle length. But you don't have a rifle gas, you have a carbine gas.

So imagine your bolt cycling back at a force that is compensated by the wrong lower resistance, there is still substantial force left at the end of the system that needs to be dissipated, which it does by transfering it rather forcefully onto the end of your tube, so your bolt, tube, buffer weight, spring, will suffer wear that it was not intended to suffer.

If the accuracy gains are worth it, and you never intend to sell it later on without disclaiming that fact, its your rifle...
Click to expand...

I'm certainly no expert, but it sounds like you're describing a weaker spring instead of a stronger spring... I may be misinterpreting though.
 
Sounds like you should order a new carbine spring and see if the accuracy stays good. Maybe your spring was just worn out and needed to be replaced, but as mikethebike said, you don't want to wear or damage other parts of your gun unnecessarily.
 
JR Hartman said:
the spring is stronger now than it was.
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your PE on that spring reaches its maximum at a longer length than the PE max position of a carbine spring, therefore there might (most probably) be KE left to dissipate, that KE has no place left to go than the compressed spring material itself, buffer weight, bolt, the tube and by extension your shoulder.

If an engineer can chime in that would be nice, thats the extent of my physics knowledge.
 
A couple of things are probably at play.

The first thing is the extra power of the spring in increasing the moment of inertia with the rearward movement of the bolt on cycling. Basically, your bolt is staying locked up a fraction of a millisecond longer, which affects the rifles timing.

The second thing that is likely happening is that it is a slightly softer shooting rifle, and just seems a tad smoother. Smoother recoil and reductions in recoil most usually improve shooter follow through and reduce flinching and other marksmanship ailments.

It is possible the change to the rifles timing is aiding in the accuracy gains, however it is probably a combination of better timing and shooter follow through/marksmanship that are seeing groups being improved.
 
beltfed said:
The first thing is the extra power of the spring in increasing the moment of inertia with the rearward movement of the bolt on cycling. Basically, your bolt is staying locked up a fraction of a millisecond longer, which affects the rifles timing.
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"Moment of inertia" refers to rotational motion, not linear motion :)
 
Ok - does your bolt not rotate?;)

Anyhow, I may be off on the technicality of nomenclature, but for sure a spring with more energy built into it will slow the unlocking of the bolt, and affect the timing.

I've not seen evidence to suggest timing alone will affect accuracy, other than the usual anacdotal post on the Internet. However, it can make for a more pleasant shooting experience, ease the beating of brass and shooter, so there certainly is benefit to be gained - just to what extent, ymmv.
 
beltfed said:
Ok - does your bolt not rotate?;)
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I suspect you are right on the first part beltfed. As for the second part, that would make sense, but it would only be a small amount. I am not a really high level shooter, but I am fairly proficient and the bit of extra recoil on this rifle won't effect me enough for what is being observed. its like softball or at best baseball sized groups vs loonie/toonie sized groups.
 
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