300 BO chamber ?

[Jonny2guns]

So I am trying to load some subsonic rounds for my 300BO, but I've tried a couple different bullets and when seated to listed COAL, the rounds will not plunk test in my gun. Even tried seating them a lot deeper and the bullet hits the rifling before the bolt closes.

I don't remember the manufacturer of the upper but I got it from Midway. It's stamped 300BO 1:8

 

[Blue68f100]

Have you shot any factory ammo through this barrel?

Confirm the barrel is chambered for 300BO.

 

[Jonny2guns]

Not factory ammo, but 150gr. spbt load and shoot just fine.

 

[Another_Kelley]

Homemade brass? Necks could be thick.... What bullet are you using?

 

[AJC1]

The leede on a 300bo is normally only beat out by Weatherby cartridges. If your using factory 300 brass your chamber is cut crazy short. Have you trimmed your brass....

 

[Jonny2guns]

Homemade brass? Necks could be thick.... What bullet are you using?

Tried Lehigh and Braso both 200 gr.

 

[Jonny2guns]

The leede on a 300bo is normally only beat out by Weatherby cartridges. If your using factory 300 brass your chamber is cut crazy short. Have you trimmed your brass....

The brass was made from LC and yes it was all trimmed to length.

 

[AJC1]

The brass was made from LC and yes it was all trimmed to length.

Lc is one that should work. Mic a loaded round at the neck and verify... yank the bcg and use a stony point tool and see what oal to the lands are.

 

[Jonny2guns]

Lc is one that should work. Mic a loaded round at the neck and verify... yank the bcg and use a stony point tool and see what oal to the lands are.

I'll try that tomorrow, thanks

 

[EricBu]

So I am trying to load some subsonic rounds for my 300BO, but I've tried a couple different bullets and when seated to listed COAL, the rounds will not plunk test in my gun. Even tried seating them a lot deeper and the bullet hits the rifling before the bolt closes.
View attachment 1182663View attachment 1182664
I don't remember the manufacturer of the upper but I got it from Midway. It's stamped 300BO 1:8

Buy a case gauge.

 

[243winxb]

listed COAL,

https://pdf4pro.com/amp/view/subsonic-data-hodgdon-powder-company-761fa1.html
2.260" ?

Maximum loaded round neck diameter .334"

 

[mcb]

I found it was easy to slightly over crimp 300 BO resulting in a very subtle shoulder collapse that you almost could not see but would keep it from fully chambering. I backed off my crimp and made sure all my cases where trimmed to the same length.

I also found that loading well short of Max OAL of 2.26 even with long heavy subs usually worked best. I am loaded really long all cooper 220gr bullets at 2.15 OAL.

 

[243winxb]

Case forming from Lee-
Case forming dies are usually cut to size the case slightly smaller than the minimum SAAMI (Small Arms Ammunition Manufacturers Institute) dimensions, to account for the brass case's tendency to spring back after sizing.

Our full length sizing dies are cut to size the case closer to the middle of the SAAMI dimension, because that is where most rifle's chambers are made, and to size to the minimum would shorten case life. Whether or not our full length sizing die will work as a case forming die depends upon the chamber dimensions of your rifle, and how much spring back there will be when sizing the case.

Will just the sized case chamber freely?

 

[Walkalong]

Lc is one that should work. Mic a loaded round at the neck and verify... yank the bcg and use a stony point tool and see what oal to the lands are.

Yes, mic them, some LC is thicker than others

 

[Jonny2guns]

Will just the sized case chamber freely?

Yes, I have been shooting 150 gr spbt out of it with no problem. All the same batch of brass.

 

[jmorris]

Not factory ammo, but 150gr. spbt load and shoot just fine.

That would lead me to believe that it’s the bullet and not the case.

A long pointy bullet is less likely to contact the rifling than a bullet that’s still fat out by the tip.


vs



You can color the bullet with a sharpie and even see where they are contacting the rifling and keep seating it deeper until the contact subsides. At that point you can see how much shorter they actually have to be to function and decide what to do that point. Like these, note the long center round had to be tapped out by the nose of the bullet, to not get stuck in the bore. You can still see the “kiss” from the rifling on the far left bullet but it would chamber and eject, without sticking.



FWIW a case gauge won’t show you anything about lead/throat issues because they have no rifling to cut a lead into or the bullet to contact. So as long as your case is good, they can “pass” rounds that, with the bullet seated too long, will still fail to chamber.

 

[Kaldor]

That would lead me to believe that it’s the bullet and not the case.

A long pointy bullet is less likely to contact the rifling than a bullet that’s still fat out by the tip.

View attachment 1182758
vs

View attachment 1182759

You can color the bullet with a sharpie and even see where they are contacting the rifling and keep seating it deeper until the contact subsides. At that point you can see how much shorter they actually have to be to function and decide what to do that point. Like these, note the long center round had to be tapped out by the nose of the bullet, to not get stuck in the bore. You can still see the “kiss” from the rifling on the far left bullet but it would chamber and eject, without sticking.

View attachment 1182764

FWIW a case gauge won’t show you anything about lead/throat issues because they have no rifling to cut a lead into or the bullet to contact. So as long as your case is good, they can “pass” rounds that, with the bullet seated too long, will still fail to chamber.

Well said. I was just about to type the same thing!

 

[jmorris]

This is showing how to set the size die for a given AR but you can also use the same incremental process with the bullet seating die and arrive at the same spot, if you don’t have a marker.

Setting size die for AR type rifle.

www.youtube.com

 

[Jonny2guns]

It's not the neck that's getting stuck, it's the projectile. While extracting a stuck round, the case came out leaving the bullet stuck in the chamber.

 

[jmorris]

Reread #16

and that was a detail probably worth putting in the op.

If you seated the bullet “a lot” deeper and it still contacts, you haven’t gone far enough, yet. Once it does clear, you can measure the difference in thousandths of an inch and decide where to go from there but you need to quantify what it does take for the projectile to fit, before you can make a good decision on what to do next.

 

[EricBu]

I've never used the braso, but I've shot quite a few of the Lehigh's in 4 different 300 BO's. You really need a case gauge. I suspect when you formed the brass you bumped it too much, and that's allowing the bullet to contact the lands. This will be quickly obvious when you gauge it.

 

[Jonny2guns]

If you set a long bullet deeper (than called for in loading manual)into the case, how does that affect your powder charge (also called for in same manual).
Example 200gr bullet 10.5gr H110 and COAL 2.26". But now seating the bullet to 2.10" COAL.

 

[mcb]

Back off the powder charge. Loading shorter than stipulated in the recipe will increase peak pressure. Making that big of change in OAL I would go all the way back to the staring charge. In this specific case given your going for a subsonic loads your pressures will be well below Max pressure for the chambering so a change to OAL that big will probably not be dangerous. If you can find other data where someone has already used that powder and bullet loading shorter would give you more confidence with the change.

 

[AJC1]

If you set a long bullet deeper (than called for in loading manual)into the case, how does that affect your powder charge (also called for in same manual).
Example 200gr bullet 10.5gr H110 and COAL 2.26". But now seating the bullet to 2.10" COAL.

Download grt and model it. No reason to guess.

 

[Jonny2guns]

Download grt and model it. No reason to guess.

What is GRT? Tried to search but came back 0 results.