Issue with short stroking

[trophyhusband]

The rifle is a Bushmaster I ordered from Cabelas last December right after Newtown. I've never been able to get it to shoot better than about 4 MOA no matter what factory ammo I shot with it. I even sent it back to be looked at. They shot a 3 shot 1.75 inch group at 50 yards and sent it back to me saying nothing is wrong with it. I would have to take it apart to see if the buffer is stamped.

As far as the lower for the 6.5 is concerned, I have used it on a friend's 6.5 upper and it cycled fine. For what it's worth, it also worked fine with my upper shooting m855. I did a lot of research for every part I bought for this lower. Now I can't remember what buffer and spring I got but I do remember that I made the best educated guess as to what would be the best match for the 6.5 knowing that it may have to be changed.

Here's the upper that I was told would be done "late November at the earliest" so I should get it any time.

Alexander Arms Online Store - 6.5 Grendel 24" Overwatch-Complete Upper Receiver - Uppers by Alexander Arms®

 

[trophyhusband]

There's something else that is bothering me. With the first round of testing, my load of 22.3 grains looked about like the second pic I posted above. For the second round of testing I loaded rounds of 21.8, 22.0, and 22.2. In this case the 22.2 shot a much tighter group, although not as great as the 22.0 from the first day. Can the different buffer and spring effect what load wold be more accurate?

 

[ICANHITHIMMAN]

There's something else that is bothering me. With the first round of testing, my load of 22.3 grains looked about like the second pic I posted above. For the second round of testing I loaded rounds of 21.8, 22.0, and 22.2. In this case the 22.2 shot a much tighter group, although not as great as the 22.0 from the first day. Can the different buffer and spring effect what load wold be more accurate?

I'm trying not to contradict myself here and keep this all strait in my head, so bear with me. To answer the above question, I would say yes, based on my explanation from before in regard to bolt lock up. That said, consider the fact that your essentially changing out your rifle parts as you do your load work up, this has to effect something.

I am a little fuzzy on what your rifle is no at this point and why you were swapping lowers to begin with?

I was just reading back and I see you have a 16" barrel, with what I will assume is a carbine length gas system. When you swapped that upper with a lower configured for a rifle length spring and buffer, you over stressed the ability of your uppers gas system to operate the bolt of your rifle. The 16" barrel with a carbine length gas system and carbine sized gas port will not generate enough internal pressure to effectively unlock and lock the rotating bolt on your weapon, thus leading to your short stroking and possibly effecting your group size.

I would suggest starting over, you have to many variable's at play to effectively diagnose your issue. Return the rifle to the configuration it was in when you got it. Then we can go over the rifle and correct any issues it had from the factory, to help you eliminate any reliability and accuracy issues you have. I will tell you right now, based on your rifles manufacture that your bolt carrier key is not properly staked and that will in my experience most likely hold true for your butt stock retaining nut as well, but they are both issues you can fix at home. I will also go ahead and say your factory spring and buffer are junk! It will not be marked, if it is I will be shocked! Take the spring out and measure it, if its between 10 1/16 Inches (25.56 cm) minimum and 11 1/4 inches (28.58 cm) maximum, then its good to go, if not replace it with one of known quality. Then look at your buffer if its stamped it should say H, but if its says H2,3,4,5 etc. your ok as well, they get heavier as the number increases. If its not stamped then replace it with one that is, they are usually sold together and I would suggest one from Bravo Company USA.

I know I'm getting long winded, next lets consider the rifles purpose. You bought an off the shelf Bushmaster carbine, they usually come with a 1-9 twist, 223 Remington chamber and standard carbine hand guards, that means your have to be cognizant in your choice of bullet weights and charge weights. NEVER USE LOADS DESIGNATED AS 5.56 NATO IN YOUR GUN, your chamber is to tight to handle the pressures of 5.56 NATO safely. Your rifle more than meats its requirement's as far as accuracy goes, can you make it better, hell yes! Consider a new hand guard, one that allows the barrel to free float, do a trigger job, either by shooting the rifle or polishing the internals. oil the crap out of your bolt and bolt carrier, I use Mobil one but any synthetic motor oil is a good choice.

 

[trophyhusband]

I am a little fuzzy on what your rifle is no at this point and why you were swapping lowers to begin with?

At this point I am back to the original configuration of the off the shelf carbine with a collapsable stock. The reason I was swapping is because I'm building what I'm hoping is a very accurate 6.5 Grendel using premium components. I have the lower completed but don't yet have the upper. I thought that the much higher quality lower would give me a far superior base with which to develop the .223 load. I thought that the load a rifle shot best would be determined by the chamber and barrel. I didn't think about the buffer effecting the load.

I would suggest starting over, you have to many variable's at play to effectively diagnose your issue.

I'll do just that.

I will tell you right now, based on your rifles manufacture that your bolt carrier key is not properly staked and that will in my experience most likely hold true for your butt stock retaining nut as well, but they are both issues you can fix at home. I will also go ahead and say your factory spring and buffer are junk! It will not be marked, if it is I will be shocked! Take the spring out and measure it, if its between 10 1/16 Inches (25.56 cm) minimum and 11 1/4 inches (28.58 cm) maximum, then its good to go, if not replace it with one of known quality. Then look at your buffer if its stamped it should say H, but if its says H2,3,4,5 etc. your ok as well, they get heavier as the number increases. If its not stamped then replace it with one that is, they are usually sold together and I would suggest one from Bravo Company USA.

I'll take it apart today and get back to you on this.

know I'm getting long winded, next lets consider the rifles purpose. You bought an off the shelf Bushmaster carbine, they usually come with a 1-9 twist, 223 Remington chamber and standard carbine hand guards, that means your have to be cognizant in your choice of bullet weights and charge weights. NEVER USE LOADS DESIGNATED AS 5.56 NATO IN YOUR GUN, your chamber is to tight to handle the pressures of 5.56 NATO safely. Your rifle more than meats its requirement's as far as accuracy goes, can you make it better, hell yes! Consider a new hand guard, one that allows the barrel to free float, do a trigger job, either by shooting the rifle or polishing the internals. oil the crap out of your bolt and bolt carrier, I use Mobil one but any synthetic motor oil is a good choice.

I'm thinking this gun will have two purposes. It will have a 4x scope on it. I was able to hit a 12x12 steel plate at 300 yards so I figure it will be at least sufficient for coyotes and maybe fox out that far, although once I get everything dialed in I'll practice with it to see exactly how far out I can be effective with it. It will also be for shorter range plinking and filling the air with lead should the urge hit me (and if I can get ahold of some cheaper bullets).

The barrel is 1-9 twist and it is stamped 5.56 NATO. I've read that factory 5.56 ammo isn't super accurate and that shooting factory .223 in a 5.56 chamber can also decrease accuracy. Now I've got a plenty of brass that has been fire-formed to my chamber so those things shouldn't be an issue.

A Ranger friend of mine that does a huge amount of shooting tells me that he has also experienced better accuracy with a 1-9 twist barrel using slower ammo so I think I'm in the right range for the best load.

I do want to do something with the trigger to make it better, but I don't want to drop the money on an expensive trigger for this gun. I don't care for the factory trigger.

Unless it's going to make a huge difference, I probably won't freefloat the handguard.

 

[trophyhusband]

I will tell you right now, based on your rifles manufacture that your bolt carrier key is not properly staked and that will in my experience most likely hold true for your butt stock retaining nut as well, but they are both issues you can fix at home. I will also go ahead and say your factory spring and buffer are junk! It will not be marked, if it is I will be shocked! Take the spring out and measure it, if its between 10 1/16 Inches (25.56 cm) minimum and 11 1/4 inches (28.58 cm) maximum, then its good to go, if not replace it with one of known quality. Then look at your buffer if its stamped it should say H, but if its says H2,3,4,5 etc. your ok as well, they get heavier as the number increases. If its not stamped then replace it with one that is, they are usually sold together and I would suggest one from Bravo Company USA.

Is the "bolt carrier key" the same thing as the gas key? if so, both screws are staked twice. The butt stock retaining nut is not staked.

The spring is 10 3/4 inches. The buffer is not stamped.

 

[ICANHITHIMMAN]

Is the "bolt carrier key" the same thing as the gas key? if so, both screws are staked twice. The butt stock retaining nut is not staked.

The spring is 10 3/4 inches. The buffer is not stamped.

They are staked, great but how well, and yes they are the same thing. Also not to nock you friend at all, but just because someone served in the military, regardless of what ever tabs they may or may not wear on there shoulder, does not make them and expert in anything. Just for reference as to what I am saying, in Army boot camp they do not even teach them how the weapon works anymore, they teach them to clean and lube it and they get a week at the range, zero and qualify that's it, that's "basic". This is also one of the contributing factors that has lead to the death of many good men and woman from the systems inspection, first in Vietnam and now again at the battle of "whant", just to drive that point home in 2003, I unf--ked a "recon marines" pig as we were about to walk out the gate, because he had fed his rounds into the weapon upside down and backwards, and for the life of him could not understand why his weapon was locked up on him. Then again in 2007, right as we were about to roll out the gate, the gun truck with the convoy commander in front of me radioed convoy wide that there primary weapon was down. Now we just fell in on this convoy for movement, never met these guys before 3rd id "infantry types". I saw 3 or 4 guys up there picking at the M2 like monkeys trying to force the charging handle to the rear. The Major was having a fit about his time table and it wasn't helping! I jumped on the truck to find a young "buck sgt" behind the gun, he told me that he had very little time on the weapon, I took one look at it and asked him if it was his weapon, because it looked like hell! I don't think it had been serviced in awhile, as this inside and the outside had at least a 1/8 of crud all over them, he said his squad leader gave it to him just minutes before, I said your squad leader F---ed you and when we get back you need to point him out to me. It took about 5 second to see the issue, I had to rip the weapon apart and re assemble it, as whoever the assclown was that had signed for the thing, no doubt had not been trained to the level he should have been. There were parts that were not installed and laying on top of the weapons bolt. I mentioned these two incidents only to illustrate a point and again not to nock your friend, these two groups of me represent the "shooters" in the US military two guys who should know weapons inside an out because that's there job. Also ranger school is a leadership school, yes its hard and yes they are elite, however not all rangers serve in special operations, or even the infantry for that matter.

 

[trophyhusband]

I understand what you're saying, but that's not an issue here. This is someone I trust and whenever he doesn't know the answer he is the first to say so. In this instance when I told him about better accuracy with lighter loads, he said he has experienced something similar. I realize that this is all anecdotal and is in no way definitive, but it does tell me that my experience isn't unique.

That said, I have more questions here. Would I be correct to assume that a heavier buffer will give more consistency in the cycling of the bolt carrier and more consistency in the muzzle velocity? Being that my rifle at this point appears to prefer lighter loads would I get a short stroking again if I went with too heavy of a buffer? In a carbine system, how much difference in accuracy could I even expect from a different buffer?

 

[ICANHITHIMMAN]

I understand what you're saying, but that's not an issue here. This is someone I trust and whenever he doesn't know the answer he is the first to say so. In this instance when I told him about better accuracy with lighter loads, he said he has experienced something similar. I realize that this is all anecdotal and is in no way definitive, but it does tell me that my experience isn't unique.

That said, I have more questions here. Would I be correct to assume that a heavier buffer will give more consistency in the cycling of the bolt carrier and more consistency in the muzzle velocity? Being that my rifle at this point appears to prefer lighter loads would I get a short stroking again if I went with too heavy of a buffer? In a carbine system, how much difference in accuracy could I even expect from a different buffer?

I have no idea what you could expect, you will gain consistency and reliability.