Increasing Freebore

[sqlbullet]

I recently built an lr-308 pattern AR10 in 358 Winchester. My goals for this build were a guide gun style carbine in which I would shoot heavy for cartridge cast lead boolits. Weight and length were important considerations. My final gun is 6lbs 11.9 oz.

The barrel is a KAK industries 16" in stainless steel. I have function tested the rifle with Horndady 200 grain soft points, and it runs just fine. The chamber is very tight though.

Coming to my problem...After the basic function test I loaded up some 298 grain hollowpoints I cast from NOE mold 360-310 (http://noebulletmolds.com/NV/product_info.php?cPath=35_300&products_id=2296&osCsid=jhi3vv7ce78ot0r1n33ll4gra4) with the hollow point pins installed. The boolits were powdercoated and then loaded crimped in the first lube groove. This maximized case volume and left rounds that would still fit in a p-mag.

Then I tried to test chamber a dummy round. No go. The bullet nose was hitting the rifling long before the action was closed. To make the issue more visible, I created a dummy round with an un-coated boolit, and cycled the bolt until it fully chambered:

165961

As you can see, i have lots of rifling engraved on the nose of the bullet. Measures a bit over .300".

My first thought was to just shoot jacketed bullets until the throat eroded, and then clean it up with a throat reamer. But, when I tried loading 225 grain Sierra SPBT to Hodgdon spec COAL for BLC2, those rounds also won't chamber.

This leaves me with a gun I want to shoot heavies in, but only works with 200 grain or lighter jacketed bullets. I could probably get an NOE 360-230RF to work with this barrel too, but still that is a lot less bullet than a 298.

Is it advisable to increase the freebore by .300"? That seems like a BUNCH to me. But, my vision for the gun is certainly boolits that would fill that freebore. I have done some chamber reaming before, rechambering a Para P16-40 to 10mm. I am only a bit nervous at sticking a reamer in my new $250 barrel.

Things I already get: Yes, a 298 is a big bullet for 358 Winchester and velocity will suffer, especially in a 16" barrel. I am kinda looking at this as a big brother to my 300 blackout. I might even try loading them sub-sonic if I can get the action to cycle. In fact, with that in mind I used a low mass titanium BCG, which also helped a ton (actually about 10 oz) with my weight target for the rifle. I am fine with only getting 1900-2000 fps with full power loads.

Yes, especially for subsonic loads I could seat much deeper in the case and not have to mess reaming my new barrel. And that is probably what I will do it the general consensus is that increasing the freebore is a very, very bad idea. However, it seems my Remington 700 in 300 win mag came from the factory with about .250" of freebore, and it shoots just fine.

So, I just wanna hear if this is definitely the wrong thing to do, or if it is an OK try and see.

What say you cast boolits?

[B R Shooter]

It's your barrel, if you want to shoot those bullets, then you'll have to throat it until the loaded case closes.

[Jackpine]

The downside that I see in extending the throat is that you will probably then not be able to seat the shorter bullets so that they will engage the rifleing. As long as you are not losing needed powder capacity, and it sounds like that is not an issue, why not just seat the bullets deeper. This is assuming that you have full bullet diameter to crimp into, and it appears that you do. I crimp less cast bullets in the crimp groove than in other locations to get optimum overall length. I almost always use Lee Factory Crimp dies. My assumption is that it will not deform the bullet. Have not tested to see if I can find any difference, but it makes me feel good.:bigsmyl2:

Good luck,

Jackpine

[sqlbullet]

Thanks Jackpine. I appreciate the feedback.

To clarify: I might explore subsonic with this bullet. I certainly do plan to load it to it's potential as well, hence my apprehension at deep seating. I would really like to see what I can get up to with a 16" barrel.

Also, while the bullet is large enough to interfere with rifling, it is not quite full diameter either. An FCD would allow me to get a good crimp .300" deeper, but it would probably also wear out the case mouths faster due to extra working of the brass.

I appreciate the feedback, but this would be a secondary option if there were a hard downside to more freebore if there is a reason other than loss of accuracy with lighter bullets.

Sounds like there isn't. At least I hope not since the reamer will be at my house Thursday or Friday!

Thanks again for the feedback, and if anyone else has any thoughts, go ahead and let me know.

[pietro]

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If shooting cast boolits is intended, IIRC, your rifle might require constantly dis/re assembling the gas system after each time it becomes clogged with lead.


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[KenH]

With his intended powder coated bullets the gas system will clog up MUCH less than with lubed cast bullets..... Not sure PC will clog any more than jackets

[gnoahhh]

Another approach would be to run a throating reamer in with a 1 1/2 degree included angle taper. Then use the same reamer to make a nose bump die to reconfigure the nose of the bullet to match the taper of the new throat. It won't effect use with jacketed bullets, in fact it may help with accuracy with them too.

[Ernest]

Why not get a custom mold casts the nose riding section small enough to bore ride with out engraving.

[flounderman]

before I did anything I would check to see if that heavy a bullet will stabilize at subsonic, or at maximum.

[leadman]

A few questions. Have you slugged the bore to find the land to land diameter just ahead of the chamber? What is the diameter of the nose as cast? Is the nose expanding when seating?
I have had softer boolits expand the nose when seating and crimping in the same operation. I would think with the hollow point this might be easy to have happen also.
I have had a couple of NOE molds that were oversized in nose and body and gave me problems. I also noticed several other members posting this also. Al might have a mold from another run that is smaller.

[brettb75]

I agree with earnest. Before I messed with a dreamer setup I would have mountain molds make up a partial bore riding section. That way the rifle will be more versatile. I don't have much so I'm always looking For versatility lol

[Fishman]

As someone with a 300 blackout that exhibits the same issue, I feel your pain. The advice I got recently was to use a throating reamer to open it up. I haven't done it yet since I am working on other projects. But I tried to address the problem by purchasing a different mould, and when that didn't work I bought another. That worked ok, until a change in alloy fattened up the nose and bit me . . . Hard. 300 rounds that wont chamber after loading a proven load, what a pain. I say ream and be happy.

[sqlbullet]

Update and couple of question answers.

Earnest: I already have a mold I like. It casts the bullets I want to use in this caliber. In order to get close to a similar mold via mountain molds plus the cost of custom hollow point conversion, I would have to spend more than a new barrel would cost. Further, the long nose is a concern as it is "unsupported". Yes, I drew up the bullet on their online tool to see.

Flounderman: Not sure how to test that if I can't seat and fire a round. Bullet stabilizer calculators indicate that I am in good shape for the proposed velocities. Well above even the marginal threshold for bullet stability. You do identify indirectly my bigger concern here. I am going to modify a barrel before validating that the AR-10 platform will function with the proposed loads. Again, I don't know a way to test that other than to forge ahead with a reamer. The cost of what I have now is sunk, and has no value to me as it sits since it doesn't meet my objectives. So, the cost of a reamer gets me testing and tweaking towards the gun I envision.

Leadman: I have slugged the bore. I will be honest that I don't recall the land to land reading. I have also verified that this is not an issue of the nose swelling during seating by taking an as cast boolet and pressing it into the muzzle. The nose is just bigger than the lands. Initially I had the issue using powder coated bullets and thought the couple thousandths the PC added was contributing to the issue. It was at that point that I verified with as cast bullets that the nose was larger than the lands.

Light for caliber bullets were never a high priority for this gun, and if I did want to use them, it would not be for pinpoint accuracy at long range. If I loose 1-2 minutes of angle accuracy that is just fine for short ranges I would use sub 200 grain bullets. 200 grain bullets I can just seat long if need be.

Also, keep in mind that I also can't load 225 Sierra BTHP to Sierra load data and have them function. The freebore is just plain cut short in this barrel in my mind.

Fishman: Thanks for the feedback. That sucks for you. FYI, I ordered my reamer from Pacific Tool for $66 + about $8 shipping. Hope it will arrive this week. I will let you know how it goes. Perhaps my experience will give you some data about how to address your blackout issue.

Thanks for all your responses. It gives me good things to consider. Just cause I talked my way past everything but reaming doesn't mean the journey wasn't really useful to me! You guys are great.