300 PRC chamber measurements

[FLguy55]

Got a question for all my 300 PRC rifle owners that have measured their chambers for load development. I know there's a 64 page thread about 300 PRC but I've scanned as much of it as I can and cannot fine my answer, and it seems like 60 pages of it is folks commenting on how bad they want a 300 PRC and don't actually have one....

So I got a new 300 PRC barreled action and put a few boxes of 225 ELDM factory down it. Now having brass, I've started measurements to work up a load but have run in to what I feel like is a problem. Grabbed my OAL gauge and my 300 PRC modified case and my CBTO measurement is only coming in at around 2.67" and straight away even visually you can tell the bullet is sitting oddly far in to the case. A Hornady factory 225 ELDM cartridge measures around 2.85" CBTO. If I forcefully jam the bullet in to the rifling using my OAL gauge & modified case, after feeling the bullet first touch the rifling, it gets to about just over 2.9" but it's clearly running hard and far in to the lands. I hear about guys loading their CBTO to around 2.844 to 2.888, which lines right up with Hornady's factory loads, but if I were to do that in this chamber that puts me .2" in to the lands which is an awful lot! I'm used to loading my 6.5 CM .020 off the lands for instance. Even if I were to load this 300 at the lands, according to my measurements, it would put my COAL at only 3.485" which is actually below SAAMI minimum COAL for 300 PRC which is 3.575". If I were to load one of my 250 grain A-tips (which I also got a box of) just to the lands, a ton of that bullet would be down in the case. I'm pretty miffed. I see guys here talking about loading their 300PRC to the lands in their rifle, with the same 225s, at 3.7 COAL. If I were to just go by that COAL I'd be .2" in to the lands which is massive. According to these measurements, I'm clearly jamming even factory Hornady rounds in to the lands quite far (which I've verified with the help of a little magic marker). This doesn't seem right to me.

What are others' CBTO measurements in your 300 PRC chambers?

 

[918v]

If you have shot factory ammo through it and your Hornady tools are telling you that you’re .200” in the lands, then your tools suck.

 

[918v]

Take a fired case and f/l size it so you end up with .002" of headspace, just enough to close the bolt easily. Cut a slit through the neck and into the shoulder. Deburr the slit. Insert a bullet about .100" into the neck. Push it into the chamber and let the rifling seat the bullet. Eject it using a cleaning rod from the muzzle end. Measure the oal.

 

[FLguy55]

If you have shot factory ammo through it and your Hornady tools are telling you that you’re .200” in the lands, then your tools suck.

Math is telling me I'm .2 in to the lands, plus the scuffed marker on a factory load when I chamber it. As for cutting up a fired and resized case, I'm using a modified case that I verified dimensions against one of my FL resized fired cases so I've essentially already done it.

 

[Supersubes]

One thing is for sure, there’s no way you “forcefully jam” a bullet .2 into the lands without a hammer. That bullet doesn’t come out without a hammer either. Chambering factory ammo would require the same hammer, because the cam surfaces on the receiver lug abutments don’t have .2” of travel. In other words, they bolt wouldn’t even begin to close because the lugs would still be in the raceways. Show us the scuffed factory round please. Any reason to believe your chamber is not a saami chamber?

 

[918v]

Math is telling me I'm .2 in to the lands, plus the scuffed marker on a factory load when I chamber it. As for cutting up a fired and resized case, I'm using a modified case that I verified dimensions against one of my FL resized fired cases so I've essentially already done it.

You don’t know what you’re talking about. Can you comprehend .2” of bullet diameter being engraved in the lands? You can’t accomplish that with finger pressure. You are not measuring correctly and your tool is a piece of shit.

 

[spife7980]

Take the personal touch out of it and let your chamber tell you.

 

[8nbait]

Most Hornady bullets are oversized. In the 300 prc chamber the throat is very tight. You will physically have to push the bullet hard until it stops at the lands.

What you are doing is correct and that will be a somewhat accurate measurement to your lands.

Berger bullets will be easier but there is still slight resistance while trying to find the lands.

The above method won’t work most of the time with the prc chamber and Hornady bullets, the throat is too tight.

 

[Supersubes]

Most Hornady bullets are oversized. In the 300 prc chamber the throat is very tight. You will physically have to push the bullet hard until it stops at the lands.

What you are doing is correct and that will be a somewhat accurate measurement to your lands.

Berger bullets will be easier but there is still slight resistance while trying to find the lands.

The above method won’t work most of the time with the prc chamber and Hornady bullets, the throat is too tight.

The saami chamber has a .3088 freebore diameter. I’ve never measured a 30 cal hornady bullet that’s anywhere near that. He should have zero interference.

 

[8nbait]

 

[MBTcustom]

Brother, I've seen this before. You've got a rough chamber. If the throat is not cut cleanly, your results will be muddy and everything you describe happens. Shoot a round of Tubbs bore lapping bullets through it, and it'll all clear up for you.

 

[Bandit320]

My CBTO measures 2.981“ in my Howa 1500.

 

[2aBaC̶a̶]

My 300PRC Oacl
3.68 225eld
3.7875 with 250 Atip
Forget exactly how far off the lands but i usually give it a generous jump.