How do you decide OAL?

I load to magazine length for the AR unless it is a light bullet that calls for less than magazine length.. Hornady 55 gr is a magazine length bullet ...

First you can look at the SAAMI dimensional drawing of the cartridge. The cartridge length is 1.760" -.030 so between 1.730" and 1.760" to be within SAAMI specification. Most reloading manuals will give a suggested trim length of around 1.750" which is .010" below the max case length.

Now we have the OAL which becomes a function of the bullet length and what will fit in the chamber. Different bullets are loaded to different OAL (Overall Length) based on their individual characteristics.

Trivial point here, you mention a 8:1 twist but I believe you have a 1:8 or 1 turn in 8 inches of barrel length. OK, that aside you are loading 55 grain Hornady FMJ BT bullets with a cannelure. The suggested COL should be about 2.200". That is for the Hornady Bullet using the Hornady 9th edition manual. Now if that was a Sierra 55 grain FMJ BT the suggested COL would be 2.250" according to the Sierra 50th Anniversary edition manual. Both bullets are 55 grain but... different designs and I would venture slightly different length and shape.

Ultimately you decide OAL by using the load data for the bullet(s) you are loading. Once your hand loading skills develop then you can begin working your own loads and bullet seating depths, till then I suggest the loading manual is your best friend.

Ron

OAL isn't really something you decide upon. Max OAL has already been determined by SAMMI and CIP(European standards guys). Absolute max OAL is 2.260", according to my old Lyman manual.
You may be thinking how does one determine the 'off the lands' distance(Mostly an over rated, mostly unnecessary number). That's a trial and error thing. Usually doesn't apply to a semi-auto. More of a bolt action target rifle thing.
However, two identical Anderson rifles with Faxon 16 inch barrels will have slightly different chambers. Easiest way to figure out the 'off the lands' thing is to loosely load an unpowdered cartridge with the bullet seated long(as in past max OAL). Chamber it gently so the rifling pushes the bullet into the case. Then start trying different lengths that are less than that length. Mind you, like JimKirk says, mag length is far more important for a semi-auto.

No matter what the SAAMI max length may listed to be,
Start w/ the OAL as established by the Bullet manufacturer.


The reason is that different bullet shapes will hit the lands at different lengths:
To wit:


In your case (Hornady 55gr/FMJBT), Hornady has already put a cannelure
at/near where you should seat the bullet. [ Hornady says 2.200" (?) ]

Given you are actually measuring offset, set for an initial
20-thou off the lands and adjust in/out from there.

JWH321 said:

Ron -- Thanks so much for that. It helps immensely. I really trying to understand what I'm doing here. I've reloaded handgun ammo for years, but this is my first try at rifle stuff. Its brought on my my recent acquisition of an AR -- something I'd never thought I'd own. Itried the 2.2 overall, but that wound up putting the cannelure below the neck of the case -- case length is exactly 1.755". My crimp is not very tight so that may have lead to the neck being a bit longer than optimal. So far I'm just loading dummy stuff trying to understand my dies and stuff like that. I have H335 sitting here on the sheld, and CCI small rifle primers -- I've seated a couple of them in order to get my seating die exactly where I want it. With help from the Lymans Manual, 49th Edition, and a lot of reading here and elsewhere on the web, I am learning. But I'm old, so I learn slowly and I ask a lot of questions.

I did mark up a bullet with machinist's blue and loaded it lightly in the chamber to give me at least a ball park baseline for ogive. When I pushed it into the case with the BCG ( I took the ejector and extractor out of the bolt), it resulted in an OAL of 2.3145 inches. 0.114 inches of gap remained between the bottom of the cannelure and the rim of the case. It measured 2.146 from the base of the case to the touch mark in the machinist's blue. .567 inches of the bullet remained above the case rim so the bullet is inserted 0.171 inches into the case at the time that the bullet contacts the rifling in the barrel. The cannelure ring is .040 wide. It follows then that a bullet seated into an OAL of 2.2 inches will result in a gap in the chamber between the tip of the bullet and the rifling of .113 inches. A bullet seated at 2.25 will leave a proportionately shortened gap. What I am trying to reach is a measurement of what the optimum length should be. I will likely move next to trying variable lengths between 2.2 and 2.25 to see what gives me the best accuracy from a bench rest.

PLEASE, if you see anything that I've not considered or that I'm just dead wrong on, let me know.

BTW, I did mean 8:1, but I didn't know how to write it -- I wrote 8X1 first and that didn't look right so I tried something else -- I know what I mean, but I aren't a real good writer.

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Mine and everyone else pleasure to help you out.

When the bullets have a cannelure it makes things easier but that will not always be the case. As to a crimp? You will get mixed opinions on this but when I load 223 or even 308 I rely on a good tight neck tension rather than a roll crimp for example. Some loaders crimp their 223 and some don't, I don't be they for bolt gun or semi-automatic. As to loading a specified length back from the lands? You pretty much seem to have called it. The distance between the start of the ogive and start of lands in the barrel is free bore or some call it bullet jump, the distance the bullet will travel prior to engaging the lands. There are several ways to figure it out. The RCBS Precision Mic set is one method. There are other gauges and methods and what you are doing works well. Just remember as was mentioned, if you plan to use a magazine you can only get so long with the OAL. When shooting Sierra 80 grain BTHP match I load one at a time as my loads will not fit a magazine. When loading bullets with a cannelure you seat the bullet to the cannelure. Just keep in mind that many match bullets lack a cannelure so then we seat to what the bullet manufacturer suggest. Only when we become proficient and have a handle on things do we start working to our own seating depth and changing the free bore.

Tomorrow I plan to be on the range running a bunch of 223 and 308 new loads I want to try.

Ron

Nothing to it, just seat them deep enough to fit and feed from the magazine, and make sure they aren't jamming into the lands.

GS

You might want to check your load as to whether a SR or SRMagnum primer should be used. All of my reloading manuals call for SRM be used for H335.

Keep this in mind if you have a bit of unburned powder upon ejection.

Hope this helps,
Joe's

JWH321 said:

I am reloading Hornady 55 grain FMJBT with cannelure.

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Seat to mid-cannelure.

JWH321 said:

Now I have another question. The issue of SRP vs SRMP seems to be at least in part in response to the slam-fire potential of the lighter primer.

Does anyone test primers? Would it be possible -- or just silly -- to prime a few cases and feed them through the gun without powder or a bullet to see if slam-fire can be induced?

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Funny you should ask. I ran some loads today strictly to see how several primers behaved. I ran loads in both, an AR as well as a Bolt Gun. Here was the AR load data:
Cases were all new Remington sized and trimmed to a case length of 1.750"
Bullets were 69 Grain Sierra HPBT
Cartridge OAL was 2.260
Powder was VV N-140 24.4 Grains
Primers were 1. WSR, 2. CCI 400, 3. CCI 41, 4. CCI BR4, 5. CCI 450

I fired five shots each (5 shot groups) and on each primer tested I allowed the bolt to freely slam home on at least two out of five shots. Magazine was inserted to create a little drag for the bolt. The rifle I used was one of my AR rifles with a good quality heavy barrel measuring 24". Chronograph about 15 feet from muzzle.

WSR 2869 FPS
CCI 400 2857 FPS
CCI 41 2883 FPS
CCI BR4 2878 FPS
CCI 450 2896 FPS

Generally for an AR the recommended primer is the CCI 41 made to be less sensitive. All of the primers were seated till they bottomed out in the primer pockets. With the tested primers I had no slam fires. When the bolt on this rifle goes home it does so with a vengeance too. Before I forget the barrel twist is 1:7. This rifle consistently shoots MOA and yes it is scoped, my only scoped AR. I did not remove unfired cases to see how much of a divot the firing pin placed in the primers from the force of the bolt going home. It was in the low 90s with humidity about the same.

For many years before CCI released the #41 primers we loaded with whatever we had. I did call CCI some time back to verify the #41 is a magnum primer. Just made to be less sensitive. My groups today were all MOA and I shot at 100 yards. What I found interesting was while the groups remained about the same size they did move a little on the targets.

Must you absolutely use a less sensitive primer? No, in my opinion. I view them as a nice to have. Eventually I would have maybe hit a slamfire it just didn't happen today during my shooting and testing. It was hot and humid on the range.

Can you load some cases primer only and let the bolt slam home on them? Yes, you can. The primers may try and back out but I doubt you will damage anything. Also, a slamfire has several possible causes including but not limited to the headspace of the rifle's chamber. So while I did not get a slamfire with the rifle I chose I could well have gotten one with another rifle.

Ron

Actually if you want to do a chamber casting I would thing about using CERROSAFE® CHAMBER CASTING ALLOY. That is what the stuff is made for with things like shrinkage taken into consideration. Easily heated with a low melting temperature and easily used over and over again.

So what did you come away with following your experiment?

Ron

Must you absolutely use a less sensitive primer? No, in my opinion.

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While the AR's bolt/carrier group makes an Out of Battery slamfire almost impossible,
it's never cool to put a bullet into your basement wall while test cycling a cartridge,
or even to put an unexpected round down range.
It's all a roll of the die, and the dice are loaded one way are the other

So I'll keep my Federals for bolt actions and other manual feeds,
and my CCI's will stay w/ the gas guns

On the subject of which primers, I used quite a few CCI-400s with my match ammo without experiencing a slam fire. And a lot of those were fed singly, sometimes almost entirely into the chamber, before having the bolt dropped on them. Nowadays I'm using CCI-41s or 450s for making range ammo. I've also used several thousand CCI-BR4s and a couple thousand Remington 7.5s, but have never used Federals.

As to OAL, with a bullet with a cannelure, there isn't much trick to it. Make a dummy round seating to mid-cannelure and measure the OAL. If you're under 2.26", try it in a magazine and then do a chamber plunk test. As long as the bullet doesn't jam into the rifling, you're good to go. I will note with some bullets and some chambers, it IS possible to jam the bullet into the rifling while fitting in an AR-15 magazine.

The Hornady bulk 55gr FMJ-BTs are actually surprisingly consistent for FMJ projectiles. I've gotten good accuracy with them out of my 16" Colt with a chrome-lined barrel, about 2" for 5-shots at 100 yards.

And you do actually mean a 1:8" twist barrel. It's shorthand for a rate of rifling that makes 1 twist ever 8 inches. An 8-1 twist means 8 revolutions per inch, like a drywall screw...

With a 1/8 twist you might want to load a heavier bullet which may be more accurate in you rifle.

The 1:8 will shoot everything well (down to/including 52gr match),
but it really comes into its own with 70-80gr+ bullets