.300 Whisper OAL for semi-autos?

[Garrett]

I have been loading .300 Whisper for some time for use in a single-shot rifle. Cartridge overall length has only been limited by where the rifling lands started in relation to the chamber.

I am now starting to work up some loads for use in an AR15. I am aware of the issue with the front grooves in the magazine potentially causing problems with long, heavy bullets. I am also aware that the magazines can be modified to fix this, and I will probably mod a couple of mags just for this purpose.

However, does anyone have any good data on how long the bullet can be seated and still function reliably in an AR15 magazine? Do you get to the point where the shortened round will no longer feed reliably? Or do most people just shorter the OAL (and adjust the powder charge accordingly) until it runs well in the magazine / gun?

eta: Oops! Wrong forum. Can this be moved to the Reloading forum?

[rsilvers]

You should not exceed the length of 5.56mm NATO ammo as that is what the mags are designed around.

The length is 57.4mm which is 2.2598 inches.

I personally load to 2.245 - 2.250 to ensure there is never a problem with an out-of-spec mag.

[LouBoyd]

Of course the cartridges have to fit in your magazines with a little clearance to function reliably. But what's important for accuracy is how well the loaded bullet will fit the throat of your barrel. Generally best accuracy in a subsonic is found with the bullet just touching the lands to give a gas seal without pushing the bullet further into the case. Crimping is usually not good for subsonics. How your particular barrel throat was reamed will determine what bullets work best. There is no standard which is right for all bulllets and there are lots of 30 caliber bullets with various ogive shapes.
There are three choices.
1. Find a bullet to match the throat of your barrel.
2. Have the chamber cut to match a partiuclar bullet during it's manufacture.
3. Live with whatever accuracy you get. The limiting factor in the range of subsonics usually comes from vertical stringing caused by velocity dispersion. High drag (low BCs) results in more wind deflection but that's not usually the limiting factor on useful range of a subsonic.

I personally don't like to have the shank (bearing surface) of a bullet extend inside the neck of a cartridge. I believe that causes inconsistnant bullet release. It's ok for a boattil to extened beyond the neck. But worse for accuracy is a bullet which allows blowby befre the bullet engraves. That can be the result of the seating depth or the angle of the ogive where it meets the lands. Don't assume a particular barrel manufactuer cut the thoat of your barrel to match the bullet you want to shoot unless you had the throat matched to a specific bullet and seating depth. Otherwise your task is to find a bullet which works.
I start by measuring how far a bullet will drop into the throat and see if it seal on the lands just by blowing air into the breach. VLDs with long ogives generally produce longer OALs before sealing regardless of bullet weight. Long angent ogive bullets may not seal even though They are touching the lands. Feeding from a limited lengh magazine is just one more parameter which needs to be satisfied for a semi auto or a bolt action.

The additional requirement for a semi-auto over a bolt action repeater is that the load cycles the action. That's true whether it's a gas operated, blow-back, or recoil operated action. Certainly there are barrels for AR-s on the market which make meeting all of the above requirements very difficult or impossible. That's mostly becuase there is no >standard< 300 whisper cartridge, load, and bullet combination. Probalby the 220 grain Sierra Matchking loaded to 2.26" is the closest to a "standard" as its one of the very few commercially available subsonic loads. (made by Corbon). That's not to say that's what you want to use if you're building a custom rifle or that all commercial barrels will work with that cartridge.

[Rikky Lee]

Good post lou. The vertical string issue is an important one to mention to new shooters as playing in the subsonic arena is vastly different to subsonic loads that we have become used to.

I'd be doing up a number of dummy rounds at full mag length and slightly shorter and cycling them through the rifle to get an idea of how that particular rifle functions. Remember that it is easier to tune a load to a rifle then tune the rifle to the load.

[LouBoyd]

Originally Posted by Rikky Lee

Remember that it is easier to tune a load to a rifle then tune the rifle to the load.

Thats certainly true if you already have the rifle. But if you already know what bullet you want to shoot and what magazine you'll be shooting from at the time you have a custom barrel made it's easier to have the chamber and throat cut to match the cartridge and bullet. Unless you provide that information to the smith, usually in the form of a dummy cartridge, how are they to know what you want? You might be wanting a dual use gun which can also shoot supersonic bullets or something.

If you already have a chambered and throated barrel You can make pretty accurate measurements with a caliper by dropping the bullet into the throat and measuring from the base of the bullet to the back of the chamber without making dummy cartridges. With that and the length of the bullet you can at least rule out the bullets which will be too long or can't reach the lands.

It would be nice if bullet manufactures would offer "sampler" sets of their bullets, Say five each of each bullet they make that you can buy by caliber with a paper which gives the bullet dimensions and recommended spin rate at various velocities for each at standard atmosphere. Life could be so much simpler with that information. You can go broke buying 50 of each bullet which "might" work, particularly in 308 caliber since there are so many.