Ultralight Ultralight Rifles

[KyleR1985]

Interesting, I think Kyle or someone got right about 2.4 with the 77s, but can't remember which lower that was. I hope I don't have to do any surgery to make the 79gr drt fit which I have on the way finally. 2.36 works well with the Tennessee arms lower without cutting anything, maybe a little longer would work

I’ll have to go back to my notes. But I think I filed my carbon lower magwell, upper, and barrel extension to allow 2.4” overall. I think I Loaded a couple TMK’s at 2.39”.


My load testing shows a much bigger increase in velocity than I would’ve expected gaining .1” of case capacity.


I’ll see what I can dig up.


For reference, this is from my wife’s Howa mini with a 16” barrel:




I’ll have to find my notes on OAL at that CBTO, but it was measured with hornady comparator.

Bolt was a little heavy at the 27gr load.

Screamin

 

[Thegman]

Well crap, it's much worse with the barrel on, regardless if there's a round in the chamber or not. Pics for you all, notice the strait-back cam action, and the one smooth (rain proof) side.
One Pic shows the bolt being forced forward but yet it still needs to go another 3/16"! It's looking like the upper is not machined correctly, but it might be that the bolt isn't able to be pushed in deep enough. I tried a lot of force, like 50lbs, but no difference, it's a hard stop. Back to BCA it's going,
Maybe i got a bear creek arse version...
View attachment 849405View attachment 849406View attachment 849407

Probably not worth messing with too much, but can you try pulling the bolt out of the carrier and removing the spring? Just trying to isolate that variable.

If the spring is too large, I imagine it could be too long when fully compressed and stop the carrier from moving far enough forward to close the bolt.

 

[Trackselk]

Probably not worth messing with too much, but can you try pulling the bolt out of the carrier and removing the spring? Just trying to isolate that variable.

If the spring is too large, I imagine it could be too long when fully compressed and stop the carrier from moving far enough forward to close the bolt.

I'm not sure. I wanted to try that, but all I can see is a small pin which looks more like it separates part of the bolt, versus removing the whole thing. I'm eager to get it replaced, so I'll hopefully have it gone in the mail tomorrow. I'm guessing they put too long of a bolt in it, as it looks to function well, but is limited prematurely. I'm afraid to start driving out pins, as they might use that as an excuse to not replace it if I leave any marks...

 

[Thegman]

I'm not sure. I wanted to try that, but all I can see is a small pin which looks more like it separates part of the bolt, versus removing the whole thing. I'm eager to get it replaced, so I'll hopefully have it gone in the mail tomorrow. I'm guessing they put too long of a bolt in it, as it looks to function well, but is limited prematurely. I'm afraid to start driving out pins, as they might use that as an excuse to not replace it if I leave any marks...

Take a look at YouTube on removing an AR bolt from a carrier. It's super simple, won't hurt anything, won't leave marks and doesn't require any tools. Takes about 15 seconds. Something you have to learn with ARs anyway, so now is a perfect time!

 

[Southernfried]

Here are some other videos to help the OP.

BCA bolt

BCA bolt pt 2

BCA bolt pt 3

Link to another forum on the BCA

Hope it was ok to list another forum, if ya want to learn AR's that is the place.

 

[Trackselk]

Take a look at YouTube on removing an AR bolt from a carrier. It's super simple, won't hurt anything, won't leave marks and doesn't require any tools. Takes about 15 seconds. Something you have to learn with ARs anyway, so now is a perfect time!

Thanks, that is ridiculously simple, I'll see what it reveals

 

[Trackselk]

@Thegman saved the day, thanks again! I was about to give up when I realized the bolt was catching on something in there, so I pulled the bolt again and saw something sticking down through the upper. Long story short, the 'guide block' that screws to the top of the carrier has 2 different length screws, and they assembled them in the wrong positions... works well, now I can roll up some ++p nas3 loads for it!
On another note, the gunsmith looked at the tiny bca barrel profile and immediately asked me if I reload hot, he was concerned... Hopefully these nas3 cases really can handle more pressure! I'm not trying to take them to the approved 80k psi, so hopefully she'll stay together. If I return missing an eye you all won't notice anyhow lol

 

[Tailwind5sw]

What does the gas key track/slot in the upper look like? Does the track widen out for the bolt to turn when the bolt handle go down or is it just real loose in the gas key base?

 

[Thegman]

@Thegman saved the day, thanks again! I was about to give up when I realized the bolt was catching on something in there, so I pulled the bolt again and saw something sticking down through the upper. Long story short, the 'guide block' that screws to the top of the carrier has 2 different length screws, and they assembled them in the wrong positions... works well, now I can roll up some ++p nas3 loads for it!
On another note, the gunsmith looked at the tiny bca barrel profile and immediately asked me if I reload hot, he was concerned... Hopefully these nas3 cases really can handle more pressure! I'm not trying to take them to the approved 80k psi, so hopefully she'll stay together. If I return missing an eye you all won't notice anyhow lol

Nice work!! Interesting issue and good to know that could be an issue.

I wouldn't be worried about the pencil barrel and the NAS cases...but then, I don't worry about much, so probably not the best guy for input on that...

 

[Trackselk]

What does the gas key track/slot in the upper look like? Does the track widen out for the bolt to turn when the bolt handle go down or is it just real loose in the gas key base?

Yup, it's widened out for that guide block and the cam lobe. That guide block appears to be bolted and pressed into place, as I couldn't remove it with my finger tips, but maybe it would pop free with a tap from a hammer. Pretty good design other than nothing keeping it from getting popped out of battery with a little push up on the bolt handle.
*I'll likely run a fat rubber band around it in the field to keep it from popping open in the brush. 10 cent fix

 

[Thegman]

Yup, it's widened out for that guide block and the cam lobe. That guide block appears to be bolted and pressed into place, as I couldn't remove it with my finger tips, but maybe it would pop free with a tap from a hammer. Pretty good design other than nothing keeping it from getting popped out of battery with a little push up on the bolt handle

I saw a video of a guy that addressed the bolt handle issue by adding a small notch in the upper to catch the bolt handle since it's under spring tension. Looks like it worked really well.

You'd have to make it as shallow as possible as it would also be letting the bolt carrier move backwards and if it goes too far it would block the hammer from hitting the firing pin.

 

[Trackselk]

I saw a video of a guy that addressed the bolt handle issue by adding a small notch in the upper to catch the bolt handle since it's under spring tension. Looks like it worked really well.

You'd have to make it as shallow as possible as it would also be letting the bolt carrier move backwards and if it goes too far it would block the hammer from hitting the firing pin.

That was on this bca bolt action?

 

[Thegman]

That was on this bca bolt action?

Yes, right here:

I see that's @Southernfried video #2 actually.

 

[Trackselk]

Yes, right here:

Hmm, there's no place for it to grab, so he cut the upper to where the bolt rotates further, which affects the bolt 'cog?' Engagement. I'm thinking this will cause excessive wear, especially with hot loads. There isn't a lot of surface area soaking up any rearward travel. What do you all think about that? I think I would be more inclined to put a slightly bigger washer under the screw, and then cut a matching recess so it doesn't affect the bolt engagement.
I'd probably have to flatten 2 sides of the washer to avoid limiting bolt rotation and maybe to fit in the handle recess for the screw. Heck, maybe 3 sides actually. So, make an oval tab basically...

 

[ACHILLES]

If you don’t want to make a notch like the video above. You might could drill and tap one of these above the rectangular piece that the bolt handle mounts to. Probably drill it in the lip of the receiver on the right side closest to the barrel. You would thread it in backwards with the ball sticking out of course.

 

[Trackselk]

One other issue I see is that the corner where the bolt handle screw head scrapes across is already showing wear after a little bit of messing with it. Looking at it closer, I see the Allen head has little 'dashes' on its side, which are working like a file on that corner. Definitely needs a smooth head, and even then, it's only aluminum, and has pressure against it from the extra spring. Luckily, if the wear gets excessive, I think there's enough meat there on the upper to attach a small steel strip with 2 tiny machine screws. You'd just have to tap the aluminum.

 

[Trackselk]

If you don’t want to make a notch like the video above. You might could drill and tap one of these above the rectangular piece that the bolt handle mounts to. Probably drill it in the lip of the receiver on the right side closest to the barrel. You would thread it in backwards with the ball sticking out of course.
View attachment 849715

I like where you're going with that, so I measured the thickness there. 5.5mm. I think it's too thin for that angle :/

 

[ACHILLES]

They make them in 4m x 6 also. Would give you .75mm walls. Not sure if that’s enough either though.

 

[Trackselk]

They make them in 4m x 6 also. Would give you .75mm walls. Not sure if that’s enough either though.

Looking closely a the contact area for this, and I think it would chew the detent ball up pretty quickly as I don't think it would align such that only the side of the Allen head would contact the ball, but with the washer removed it just might. I'm skeptical that it would give enough resistance, but it might be excellent if executed perfectly. With only .75mm to play with, and a very difficult angle to tap the threads, this would be skilled machinist territory. Maybe there's a 3-4mm plunger and spring set-up that would work? I don't know how you'd be able to limit the plunger/ball from falling out though