AR-10 in .243 win

[tangolima]

I did that. Put together with most affordable (cheap) components I could find; blemish lower + BCA side charging upper. The only upgrade is a $65 2-stage trigger made decent with a bit of work.

First the gas was shut off to turn the rifle into a straight-pull. A load was worked up. Loaded 10 rounds for a "qualifying group". First 5 rounds made solid 0.3" group at 100yd. For the heck of it, I opened up the gas to shoot semi auto. The remaining 5 rounds opened the group to 1.2". Something moved after the bullet passed the gas port hole, I think. It has a 24" barrel so the dwell time is relatively long. When come to think about it, it takes very minute muzzle movement to open the the group; +/- 0.003" will do it.

I'm going to keep the rifle in straight-pull config. Shooting slowly saves ammo, making each round counts more.

The load is with 75gr Speer varmint bullet (cheapest I could find) going at 3375fps. I like it. So far it has put my RPR in 6.5CM in shame, and it costs less than half. Scope is a $200 SWFA SS 10x44 in MOA. Kicks butt with zero nosense.

-TL

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[Logs]

Nice. I bought a complete BCA AR10 in 6.5 Creedmore and it runs great, but is heavy as heck.

I'm looking at picking up a 243 upper as well.

 

[stagpanther]

You might be having issues with feeding (when shooting gas/semi-auto). I don't mean whether or not the cartridge feeds, fires and ejects properly--rather how "gracefully" the whole process transpires so that the the cartridge makes it into the chamber with as little physical abuse as possible; it's pretty easy for the bullets to get knocked hard enough to effect concentricity, seating depth consistency etc--especially with high BC bullets seated long which approach the magazine's length capacity. I do a lot of "cold cycling" to see how easily the cartridge makes it in and out.

 

[tangolima]

The rounds were loaded in magazine and fed sorta "normal". Charging handle was pulled all the way back and let go. They weren't single loaded.

The bullet is a short flat base HP. COAL 2.58" , or 0.13" from max. The round is pretty sturdy.

The upper is indeed pretty heavy. But I thought it was because of the 24" barrel. Compared to the RPR , it is not bad. I don't plan to take the rifle mobile, so it is quite alright.

-TL

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[44 AMP]

With the gas system "on" the vibration of the rifle is different than when it is "off".

Actual movement of the action happens well after the bullet is gone, but tapping some of the gas off the barrel and it moving down the tube, is a change from what happens when you don't do that, and that alone might be enough to change the harmonics of the system enough before the bullet exits the barrel.

 

[tangolima]

Maybe. The difference is whether there is gas in the gas tube. Not sure the change in mass distribution is enough to change "harmonics".

True that the action is unlocked after the bullet exits the muzzle. But the chain of events starts before that. The bolt carrier travels back by 1/4" before the bolt starts to rotate to unlock. It is the part of the delay built in the system. The gas impulse arrives the action while the bullet is still in the bore. It fills the chamber volume. Takes time to build pressure to push the carrier back. Spring starts compression. Force on shooter's shoulder starts to change. Etc. All this happens while the action is locked and bullet still making way to the exit.

Anyway just thought it was an interesting observation to share. Will keep the rifle a straight-pull. The side charger makes it quite doable. Actually plan to modify the bolt handle to make working the action even more natural. It may become my default equipment shooting longer range.

-TL

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[tangolima]

Another observation if I may.

I had a chance to monkey with the gas block. The rifle has rifle length gas system. The gas port hole has diameter of 0.05". It is very small compared to AR-15 rifles, where gas port hole is 0.09". Difference in port pressure perhaps?

-TL

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[stagpanther]

Rifle length for 24" seems pretty short--surprised it's not rifle + 2"; maybe that's why they opted for the smaller port (sounds like maybe tuned for a non-adjustable "one setting fits all" block).

 

[tangolima]

I have the same thoughts. I expected the gas port be a bit farther out. Being an affordable brand, BCA has fewer variations in their products.

Rifle length has gas port 12" from the breech. That leaves the bore after the gas port same length as the gas tube. Gas travels faster than bullet. It is quite certain that the gas impulse reaches the action before the bullet exits muzzle.

-TL

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[44 AMP]

Something moved after the bullet passed the gas port hole, I think.

yes, something moved after you opened the gas port hole, the gas did.

Assuming we leave the shooter entirely out of the equation, and assuming the ammo is a perfect constant, and leaving out any chance wind might have played a part, what is left to look at? Only the rifle itself and the two slightly different configurations of port open and port closed.

Though one might also consider the relationship of the parts with each other and how they could be altered by the difference between manually loading the round (using the charging handle) vs. the action reloading each round under the pressure of the action spring.

Its a long observed fact shooting semi auto pistols in a Ransom Rest that the first round (manually chambered) often has a slightly different point of impact from the other rounds in the magazine, that the gun itself loads after firing.

I think that between those two possible factors it might explain the change in the group size in your rifle.

Repeat the expiriment. Do it backwards (shoot semi first then shut off the gas system) and see what happens. I wouldn't be expecting exact repeat of the measurements, but look and see if there is a discernable, and repeatable pattern.

In other words does it always shoot tighter groups with the gas off, or not?

IF it is consistent, then it seems to me the larger groups must be the result of either the gas system operation, or the rifle reloading itself, or both together (tough call which since you don't get one without the other).

 

[stagpanther]

Another thing I can suggest--people often think their AR barrels are prim and proper when shipped to them--first thing I do is inspect the bore--if it's dirty that's no problem, generally means it has been test fired, but the gas port is a real weakness in AR barrels that many manufacturers seem to overlook or simply don't care about enough to finish them properly. poor positioning can result in jagged edges and accelerated erosion of the port, tearing away at the edges and inside the port itself. That stuff (along with other junk jetted at high pressure) gets blown up into the block and tube and can possibly impede its functioning properly.

 

[tangolima]

My expectation in this case is quite on the contrary. The rifle has performed above and beyond, even with the 1.2moa semi auto group, considering how much I have paid. Never bore scoped the barrel. Didn't break it in either. Ran a couple of patches down the bore to soak up extra oil in there. It was clean.

It is a tinkering toy, and more tinkering I plan to do. Next up is 20 more rounds to shoot. If it holds up 0.3 moa straight-pull for real, the tinkering will probably take a break. It will kick a lot of butts.

-TL

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[stagpanther]

I've been looking into the AR to bolt-pull conversion market--the day may come when semi-auto ARs might be "outlawed" in which case (if it's an option) I'd like to convert over to bolt operated, otherwise I will be out a fortune invested in ARs. Everything I've seen so far is more or less a variation on drilling a hole into the carrier and attaching a knob for racking.

 

[tangolima]

A side charging upper makes it much easier.

-TL

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[44 AMP]

Everything I've seen so far is more or less a variation on drilling a hole into the carrier and attaching a knob for racking.

While this is certainly possible, it begs the question of "why?"

To make manual operation slightly less tedious than using the poorly designed AR charging handle is the only reason I can think of.

If we are legally denied semi auto operation, its a good bet we'll also be denied the standard capacity magazines (which the banners call "hi capacity), and if that is the case, I don't see anything one can do with a neutered AR that isn't done as well, or better by a regular bolt action rifle, particularly in the larger calibers.

I understand well the point of wanting to be able to keep at least some utility from one's investment, and I understand that for some of us there is more than mere money involved.