Looking for clarity on 308 case length and COL

[11B1P]

I am new to reloading and have started with a Hornady single stage press and dies. My first set of loads I used the Hornady 9th Ed and it says case trim length should be 2.005" and I used their recommended COL of 2.810" because well, I don't know any better. I used Lapua brass, IMR-4895 and Hornady 168 amax and picked the middle 3 loads Hornady recommended out of 5 to test. Not knowing any better, I then cleaned and resized the brass then measured the cases and trimmed them back to 2.005" +/-. I had no problems with any of the rounds using an AI AX308, other than the lower loads were about 3 feet short of factory ammo, but that's another story. Since then, I bought an OAL gauge that fits into my rifle and I get 2.2323" average for OAL (ogive) to the lands. I then found the saami website and looked at their data. They list 308 case length as 2.015" -2.020" and COL as 2.490" min to 2.810" max from the tip not ogive though.

I'm using the same unfired brass, but now reloading SMK 190 as well as the Hornady 168 and waiting on finding Berger 175 and 185 VLD. My questions are these. Why the difference in case length rec's? If I trim too much or too little what are the effects on accuracy and more important, my safety? Lastly, I've read ad nauseum about jump space and whether it's necessary. So, my next reload I went with COL 2.21" ogive which leaves .002" jump, but next I was going to try what I measured, 2.232". What is the downside of setting the COL to my measured rifle COL?

Thanks for your help

 

[LRDD]

You trim your cases back so they will fit in your chamber. Anywhere between 2.005 and 2.015 is supposed to be safe to fit. Jamming the bullet right against the lands adds increased pressure of the round, so I would start a bit set off and work up.

 

[haroldwilson]

Your trim length has to do with how long your chamber is. Your base to ogive dimension has to do with how long your throat is.

2.015 is the max trim length from SAAMI. It is meant to keep people from letting their brass get long enough to jam into the end of the chamber and causing a pressure spike. Pressure spike is bad.

You can buy a gage, not the gage you used to measure to the lands, and measure your chamber. Your chamber might be longer than 2.015, I"m betting it is. If you know this dimension, you can let your brass grow longer than 2.015. if you don't know your chamber length, don't let it grow longer than that. There are many opinions on how much shorter than chamber length brass is safe at.

You are not restricted to length unless you are magazine feeding. If you are single loading, you can load longer. That is where the gage you used to measure your throat comes in. If you want to jump your bullet .010, you know the base to ogive dimension to do that with. If you want to jam it, you have the dimension for that.

There is nothing wrong with setting your COL to have your bullet touching the lands or even jammed into the lands as long as you safely work up the load to make sure you aren't in a pressure situation.

Also, be aware that each of those bullets can have a different base to ogive dimension. You need to measure with your gage with each of those bullets and note the measurement to the lands. One might jam at the same length the other is jumping.

Also, in my opinion, a .002 jump is not a good idea. There can be enough variation in bullet dimensions and your measuring tools to not be able to keep that dimension stable. If you are long by .002 or more, you are in the lands instead of jumping. I think .005 is a good minimum jump. If you are jumping less than that, you might as well jam and work up your load safely for that condition. That's my opinion.

 

[mdmp5]

2.810 COL is the longest length that will feed from most magazines.
.

Most magazines? AI mags load out to about 2.9".

 

[haroldwilson]

Most magazines? AI mags load out to about 2.9".

Some mags?

Point was there is no reason you must load shorter than jam unless you are mag restricted.

 

[11B1P]

You can buy a gage, not the gage you used to measure to the lands, and measure your chamber.

Do you recall what that gauge is called?

Also, be aware that each of those bullets can have a different base to ogive dimension. You need to measure with your gage with each of those bullets and note the measurement to the lands. One might jam at the same length the other is jumping.

That's something that makes total sense that I didn't think of...

Also, in my opinion, a .002 jump is not a good idea. There can be enough variation in bullet dimensions and your measuring tools to not be able to keep that dimension stable. If you are long by .002 or more, you are in the lands instead of jumping. I think .005 is a good minimum jump. If you are jumping less than that, you might as well jam and work up your load safely for that condition. That's my opinion.

Harold,
Thanks this makes it more clear. I appreciate your response, John

 

[haroldwilson]

Sinclair chamber length gage.

 

[MisterTwoForty]

I trim my cases to 2.005 and run COAL to 2.80.

 

[11B1P]

Thanks, I appreciate your help.