Why your AR conversion won't shoot accurately

[Dolomite_supafly]

So the other day I was talking to a friend about why 22's don't shoot accurately out of an AR conversion.

We have all heard it is because of the "small" 22 lr bullet going do the "big" AR barrel. I have seen it as well as every other conversion owner. I said to myself there is probably more variation between rimfire bullets than the difference between a rimfire bore, .218"/.223", and the centerfire bore, .219"/.224".

Well that got me to thinking about just how "small" 22 lr really is. So I grabbed a handful of different 22 lr ammunition, a micrometer that went down to .00001" and began measuring. The results are quite surprizing.

Now realize the average AR centerfire barrel is .224" at the largest so anything that size or bigger than that will seal properly and will will engage the rifling. Twist has some affect on it but not as much as one would think.

Every 22lr bullet I measured was at least .224" and the majority of them was .225". Now an centerfire 223 AR bullet before being fired is .224" so all of the 22lr bullets should fill the bore as well as the centerfire bullets. I even had some 22 lr bullets that measured .226".

So what is causing your AR conversion to be so inaccurate?

It is the 1" of smooth bore the bullet has to travel before engaging the rifling.

And to go a bit farther with this. I have a 9 twist AR barrel that is used to make a 4" barrel for my Cricket. It is a centerfire barrel with the correct bore dimensions. After I built the barrel I shot it for accuracy and it was way more accurate than the conversion out of a 16" barrel. So it is not the bore dimensions or the bullet dimensions that are causing the issues, it is the freebore.

Just thought others would like to know.

Dolomite

[kwe45919]

Interesting. Glad someone looked into it. Though about getting one of the "plinkin" conversions but many have said to just buy a didicated upper because of the accuracy vs. the conversion. I think you just proved why....

[smopoim86]

What diameter in the smooth bore? Is it possible the bullet is getting swaged down too small? Press a bullet through the conversion and measure it. I'm curious.

Edited June 14, 2012 by smopoim86

[Dolomite_supafly]

I will do that.

Dolomite

[Dolomite_supafly]

It swagged the bullet from .22465 down to .22150 so that is the issue.

Dolomite

[lowery100]

I thought the extension on the bolt of the 22 conversion got the l.r. past the freeboar part of the .223 chamber. I've always understood it was the fact the twist rate is not ideal for the l.r. Most 22 l.r. barrels are 1:16 +/-

[Dolomite_supafly]

There is about 1' of smooth bore that swages the bullet down.

Dolomite

[R1100R]

Is what you are reffering to as the smooth bore inside the chamber insert??

I just want to make sure I'm following this correctly?

[Dolomite_supafly]

Is what you are reffering to as the smooth bore inside the chamber insert??

I just want to make sure I'm following this correctly?

Yes, it is undersized and squeezes the bullet down to .221". I had a friend check his and it was the same.

Dolomite

[ted]

So it would not be a good idea to get the 22lr cmmg kit for the ar15 .? Would it be a waste of money , would it be better to get a complete upper made for 22lr ? Or jest get the complete ar-15 22lr rifle ?

[Dolomite_supafly]

A dedicated will be more accurate but there is nothing wrong with the kits as long as ultimate accuracy is not you first concern. My kits have always shot around 1" at 25 yards which was plenty for what I was using them for. I used them to run drills using the same optics, same trigger, same weapon. My dedicated is treated like its own weapon.

Dolomite

[crashgordan]

so, i see a business opportunity here... if a guy sends you his conversion with return postage paid, what would you charge a man to perform the same surgery on his?

just curious.

[Kevo]

Dolamite Supafly you are the man. I've paid 10 bucks for a magazine just about AR rifles. It said the same thing about conversion inaccuracies that I've read everywhere else. Twist rate of host rifle and the smooth bore before the bullet exits the conversion. You should be a professional writer for those magazines and/or a consultant for the industry. Congratulations on becoming a moderator, Kevin

[bigwakes]

I don't have a lathe, but would be very interested in having my cmmg conversion fixed.

[R1100R]

I don't have a lathe, but would be very interested in having my cmmg conversion fixed.

Same here !! I would like to give mine another try. I have since bought a M&P 15-22 because of this. It is the most fun for the buck I have bought in a long time. I have to fight the children to get it away from them. That being said it would be nice to train with my normal rifle.

[Tempest455]

I spent about 30 minutes opening up the freebore of the conversion so the 22 bullets would drop out at .223-.224 if they are larger than that to begin with. I chucked the chamber insert into my lathe then used a 5.7mm drill bit to open it uo then followed it up with a file as it spun. Then I began with 600 grit sand paper, then 1000 and then 2000. The

I shot some groups at 25 yards and with Winchester Subsonics, which started out at .224", the holes were touching. With the SSS, which started out at .225", the group was even smaller. This was in a 12 twist gun.

Is it as accurate as my match chambered 22? No, but the groups were probably cut in half from what they were before I opened up the freebore.

It has little to do with the twist. If that were the case then most pistol caliberss would be innacurate. Most are over stabilized according the the calculations. I have also used a 9 twist .224 centerfire barrel and shot standard 22lr with great results.

Dolomite

Good info. I'm going to do this!

[deafdogdief]

Interesting stuff. So refreshing to see information from experience!