Using 16" load data in an 8" gun

[azrocks]

All load data everywhere for what I'm interested in at the moment is based on 16" barrels. But my only firearm for this caliber is 8". I'm curious how - or if - any of you venture outside published charges, or modify expectations, based on the difference. If a max charge of 20g of powder x produces 2000fps in a 16" rifle, do you try to get close to 2000 in your load workup despite going over published charge while watching for pressure signs each step of the way? Do you keep your max charge the published max charge regardless of the difference in barrel lengths & the velocity loss, and regardless of whether pressure signs are present? Does the reduced velocity inherent in a short(er) barrel translate linearly with a corresponding decrease in chamber pressure for a given charge?

 

[243winxb]

Keep the published max charge regardless of the difference in barrel lengths.

https://www.hodgdonreloading.com/re...-barreled-rifle-require-different-reload-data

DOES MY SHORTER BARRELED RIFLE REQUIRE DIFFERENT RELOAD DATA?
We often get this question. In asking, the customer is unsure if the different barrel length will necessitate a loading data change in the powder chosen. Let’s preface the answer with a quick guide on testing data.

All our data is tested for Pressure and Velocity with instrumented equipment as established by the Sporting Arms and Ammunition Manufacturers Institute (SAAMI). The dimensions of the test equipment are established by SAAMI, both internal (such as chamber) and external such as barrel length. The barrel length is set based upon most popular usage. Rifle calibers are set at 24 inch barrels. Handgun calibers are set based upon the most common use at the time, i.e. a 45 Auto barrel is based upon the barrel length of a 1911, and a 38 Special barrel based upon a standard police issue revolver.

The first thing to remember is that the chamber dimension does not change based upon application. A 223 Remington chamber is the same whether the gun it is in is a handgun or a rifle. The chamber dimension determines the pressure. So, the pressure is the same when fired in that chamber in a rifle or a handgun. The barrel length has no impact on the chamber pressure and hence the reloading data (powder charge and pressure).

The length of the barrel will change the actual velocity you observe. In general as the barrel gets shorter than standard the velocity will lower, and as the barrel gets longer than standard the velocity will increase. This does not change the reload data (powder charge and pressure), just the velocity.

Please note that there is no magic number for how much velocity is lost or gained by changing barrel length. This is because cartridges go from big to small and use varying amounts of different burn speed propellants. The only way to know for sure what the effect is with your different barrel length is to shoot over a chronograph.

So, the quick answer to the question is that the different barrel length will not change the reload data but it will impact the velocity you get.

 

[rfwobbly]

Typical setups, as explained above...




.

 

[AJC1]

Max charge is based on chamber pressure. Your desire for speed is limited by barrel length not powder charge.

 

[GooseGestapo]

Some of the data is taken from a shorter barrels, but I’m not familiar with a source for AR pistol data specifically.

Some powders may have a sufficiently different pressure curve such as to be better suited to the pistol length barrels, but again I can’t offer a specific suggestion.
A max load is a max load, pressure wise, but YOUR gun might prefer something different.

In the late ‘90’s, I had a Bushmaster AR with a 14.5”bbl and permanent AK style muzzle brake. It had a pronounced preference for IMR4198 powder for accuracy. BLC2 gave significantly higher velocities but also dramatically more flash and blast...

Several years back, the Thompson/Center Contenders were popular and several manuals had data for them. That should give you some answers for what you seek.

 

[Demi-human]

Does the reduced velocity inherent in a short(er) barrel translate linearly with a corresponding decrease in chamber pressure for a given charge?

No. Barrel length will correspond linearly to loss of velocity, with identical chamber pressure.

You can not put more pressure in the chamber to get it back. Max is max, and is measured as peak pressure regardless of barrel length. A short barrel does not lessen chamber pressure.
A long barrel does not increase the pressure, it just makes the most use of it.

Do you keep your max charge the published max charge regardless of the difference in barrel lengths & the velocity loss, and regardless of whether pressure signs are present?

Yup.

If a max charge of 20g of powder x produces 2000fps in a 16" rifle, do you try to get close to 2000 in your load workup despite going over published charge while watching for pressure signs each step of the way?

Nope.

The chamber doesn’t care what barrel length is on it, or if it even has rifling.
You will not get there with half the barrel length.
A different cartridge might, but over max is over max. The bigger cartridge with more gunpowder is still working at under a maximum pressure limit.

Pressure is what automatically disassembles rifles. They can take the burn for a long time, but over the pressure limit and...
Kaboom.

@243winxb has it right.
Load what the book says, then see how it shoots.

 

[azrocks]

Really appreciate all the info, folks! I've got it sorted out now thanks to you and yesterday's research-o-the-day.