300 BO brass not fitting gauge

[Rich22]

It doesn't make any difference what manor you use to cut the cases down. The simple fact is that you still kick up a small burr both inside and out and this needs to be addressed after the cases are cut and before they are re-formed for 300 blackout. Annealing is NOT necessary when making 300's. I really question your measuring capabilities when considering the shoulder has sprung back and I suggest a professional check a few of your cases. Anyone can pick up a pair of calipers but multiple different readings will result with people that are not capable of measuring accurately.Already I have mentioned the Lee quality issue especially with a newer caliber like the 300. Not all dies are made correctly and when it comes to something like the 300 I'll spend more money to get something reliable and accurate. I was able to do two things.... first I measured the case against the SAMMI specs and was right on and second I used blue to obtain a visual contact pattern on the cases. Both were right on. The theory about military brass not forming correctly is not true as they form and measure within a few thousands of any other case that has been formed. Headspace should be correct as long as the barrel/chamber isn't shot out and any good headspace gauge used in checking the finished 300 case should be adequate and will tell you in an instant that your formed brass will fit and function in your rifle.

So far what I have done that seemed to work after cutting I run them through a few hours in the stainless tumbler and then dry and then lube before forming. I am using the Hornady dies, not lee.

[Rich22]

Talked to Hornady for the hell of it. The guy said to run it through twice, first time without expander ball and second time with expander ball. In my two experiments so far it seemed to work, only issue is adding yet another step. If I can form on the progressive that really won't matter but I have not tried that yet.

[6bg6ga]

Thought I would try forming some cases without the decapping and neck sizing rod in the die. My results were the same. I ran a number thru the sizing die cleaned them and checked them in the head space die and I had no difference. Put the decapping neck sizing rod back in lubed the cases again and ran them thru the die again cleaned them and checked them with the gauge. The same result....all passed. One more thought....took out the 223/300 case holder off the press and replaced it with the #16 (9mm I believe) and sized a few again and my cases varied a lot with a 35% failure rate. Replaced the case holder and installed the correct one again and the cases formed correctly. With the RCBS small base dies I have to work extremely hard to get a bad case. This leads me to think there is some variation among die manufacturers and possibly all dies do not return the same results. I would encourage other to try the same experiment I have do to see if it holds true with your particular die set.

[jimofaz]

+3 on the SB die, 'specially if you are running a gas gun. Grinding back a LEE FLS sizer to get the initial fire-form case prep/Sheridan Gauge work done WILL work, but you would be better served with an RCBS SB die for this. Never had to grind any off the die bottoms and do not need any press 'cam-over' to set back the shoulders for AR reloads. Bolt rifles would not need this safety margin, as long as the bolt closes normally on a re-sized case. Cases will last longer if you do not overwork them. How much you need to rework the cases will depend on the individual rifle chamber. I size the case shoulders back approx. .002" to .003" from fired brass in my AR's using RCBS SB dies. I use a Forster headspace gauge to keep track of this measurement in my AR's, but the less costly Hornady LNL headspace gauge will also work well. This & the SB die should give you trouble-free fun. The odd fat-head mil surp M-60 brass case that goes thru the preceding treatment but still does not fit my Sheridan gauge gets dumped.

I have not found annealing to be an absolute necessary step with LC 5.56 brass. That said, I still do it because I have a Bench Source annealer to make that chore easy. fast, and repeatable. I also like the consistent neck tension annealing gives with with cast boolit .300 BO loads in my AR-15. Can't say that it helps for sure, but it sure does not hurt any.

Take your time, use good tools, be careful, & Best O' Luck with your project!

[DocSavage]

I've made boat loads of 300 blk using a Redding file and trim die and once formed Redding standard dies have resized brass that works fine in my Noveske upper.

[jimofaz]

Want to add that our own 'GRUMPA' does a bang-up job of converting 5.56 brass into ready-to-load .300 Blackout. Better quality than the one other cast boolits vendor I tried.

[strobro32]

I have had the same results after forming 5K rounds of BLK. The problem is the Sheridan slotted case gauge. It's on the < side of the specs. If a cartridge works in the gauge, it work in a chamber. Making cartridges to fit the gauge does not = making cartridges that fit your chamber. I've mage 100s of rounds that will not wok in the gauge that camber fine in my Noveske or Seekins chambers.

Also you do mention which brass you are converting. Some brass is too thick to use without turning the necks. Stick to American made brass, WIN, LC, REM and FC.

[6bg6ga]

Everything I have formed using my small base RCBS dies and checking them with my Lyman headspace gauge has functioned flawlessly in the two 300 blackouts I currently own. In my instance if it is correct in the gauge it works in the gun. Cases on the minus side have problems with non-functioning primers. I set up so the case is at the high side of the gauge and no problems.

[Rich22]

While I am thinking of it, anyone ever had a round loaded with a cast bullet of say .310 or .311 actually be able to pass a case gauge like the sheradon? So far I have loaded many that have worked in the gun but the gauge is essentially useless

[strobro32]

Bullets over .311 will not fit in the Sheridan gauge. My Lee cast bullets are sized .310. I had to size the 220 grain Leatherhead coated bullets to .311 because they came out-of-round. They fit the gauge after sizing.

[6bg6ga]

Didn't see the advantage of the gauge until I looked it up. Got to get me one of them.

[6bg6ga]

Once formed and fired in your gun you shouldn't have to size twice. I have to do that with MG fired LC brass the first time, No problem after that. An OP here that converts and sells BO brass says he has to remove 0.010" from almost every die to make them work.

I still do not understand why he is removing .010 off the case holder. My RCBS dies(small base ) seem to do a good job of sizing. The cases fit the gauge (standard Lyman head space gauge) and the bottom of the case is between the minimum and maximum for head space and the case over all is on the low side which allows me to shoot it a number of time before I need to cut it back and deburr it. My particular cases do function correctly in my AR-15 300 blackout. I feel that any error will be corrected when it form fires the round.

[Hamish]

6, if you're not using brass that's been run FA, then most likely you won't have to grind the SH down.

[trebor44]

Any issues that I had with 300 BO brass, whether home grown from 223 or commercial disappeared when I switched to the RCBS 22407 die set. My 223 based brass is multiple headstamps and I do anneal it before sizing (just my process). ALL rounds are checked with the Sheridan gauge (again my process) and all have been go to go. Prior to the switch to the SB die I had chambering issues with brass that was supposedly gauged in the Sheridan. My vote is for the SB die set. The SB die set in other calibers has been my preferred solution

[Rich22]

Looks like I have a temporary solution at least, have to run through two seperate dies to get it done but the shoulder looks good, headspace falls within the min and max of the sheridan gauge. Now just need to get a gauge that works with loaded ammo with cast bullets. The cut out gauge is just not passing anything even if it works flawlessly in the gun

[Omega]

I convert all my 300Blk cases, mostly using LC brass and Lee Dies. I polished the case holder a bit because I was having the same issue when I first started, but when I made sure I setup my dies as per directions I had no further issues except that FA fired cases sometimes need a couple passes, size, turn, size again. I got some Hornady dies to try out in my portable press setup, haven't tried using them yet but expect no issues. One thing I did find out was that there were some headstamps that were just too thick to use as is without turning the necks a bit. That is why I stick with LC and other NATO headstamps now.

[garym1a2]

RCBS small bases dies. I had lots of problems with my lee 300 blk dies.

[sparky45]

+1 on the small base dies. I get a more consistent gauge fit since going to the SB dies. And as stated above, I use ONLY LC for conversions.