Reloading for a semi auto.

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katanaa

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Hello all. This might be a silly question but I'm wondering if lower speeds are to be expected from a semi auto (such as my mini 14 target model) than what is published in the load data books. I know that the books are just guides but for everything else I've loaded the speeds are in the same ball park. But the 20,5gr of 3031 behind a 75gr projectile were around 300fps slower than what was listed.

To me it makes sense since the gas pressure will drop off before the bullet exits the barrel since once the projectile passes the gas block ports the gas has a new chamber to pressurize. But since this new chamber doesn't have a constant volume it will continue to steal the projectiles drive energy. So if that is correct then the projectile would be slowing down in the barrel once it passes those ports.

Now assuming that is correct can you load heavier loads in a semi than you would otherwise in a bolt action. Or do you just accept this velocity drop as a trade off for having an auto loaded?
 
You can prove this for yourself... close off the gas port and slowly increase powder to book listed levels.

Did the speed go to what was expected given the shorter barrel length?

Just be very very careful, that the action is fully locked up and stop if there is any sign the action is starting to unlock on its own without any gas pressure motivation.

You can hold the action shut as this is more about velocity then accuracy.

Be careful... All semis are designed around the ammo that is standard.. overloading the ammo can lead to very bad results, damage and injury.

Jerry
 
You're losing a little bit of velocity from feeding the gas system but you're losing the majority of it due to the barrel length difference. Hodgdon's published data is based on a 24" 1:12 twist barrel. Your Mini14 has an 18.5" barrel. There's close to 200fps difference from an 18" to 24" barrel for 223.

Velocity is always good but I'd say don't get caught up in velocity over accuracy. I'd rather have a load that's 300fps slower but accurate than one that 300fps faster but groups 2x as bad. Use the data based on your most accurate rifle loads to build your own dope charts.
 
I have reloaded a particular caliber that is shared between a new Troy PAR and a rifle length AR15. If I use the cartridge that is setup for my gas gun I have found high pressure signs with pushed out primers and case separation. To get the identical velocities between the two rifle with identical barrel lengths I am required to subtract a full two grns of powder for the Troy pump gun. Semies require more gas to function and is more sensitive to various powder types.

Always start 10% lower from book numbers and work your way up.
 
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As stated above barrel length has the greatest effect on velocity, also semi-autos are designed to operate at a given port pressure.
Meaning hotter loads will increase port pressure and strain on the action.


Reloading for AR15s and other .223 Rem Gas Guns
http://www.accurateshooter.com/cartridge-guides/223rem/

"With their multiple bolt lugs and rapid locking/unlocking, AR15s are more sensitive to pressure and bolt thrust issues than stout, modern custom bolt actions. There is also a risk of slam-fires in AR15s. Therefore, some high-end loads that may work in a BAT, Barnard, Borden or Stiller bolt action will be too hot for an AR15. This is why Sierra issues a completely separate load map for AR15s chambered in .223 Rem and 5.56×45 (see notes above on 5.56×45 throat length concerns). We recommend the you initially reduce loads at least 0.7 grains for an AR15 compared to a bolt action, and never exceed the powder manufacturers’ recommended loads."

223 Remington (AR-15)
http://accurateshooter.net/Downloads/sierra223ar.pdf

223 Remington (Bolt Action)
http://accurateshooter.net/Downloads/sierra223rembolt.pdf
 
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I suppose I should point out that my mini is the target model with a 22" barrel and the test gun in Hornady's book is a 19" barrel but also a semi auto. I did start back about 10% max. I actually haven't even check it for accuracy just made some for plinking on short notice. But I pulled out the chrono for a buddies gun and check these while it was set up and the results got me thinking. Thanks for the info.
 
Is the chronie working properly?

light, cold batteries, old batteries... all can have an affect on the output value of a chronie. I went to a magnetospeed as I was not getting consistent readings with my chronie.. variations of 100 to 200fps were common due to variations in light and battery.

So just make sure the chronie is working first.

Jerry
 
katanaa said:
I suppose I should point out that my mini is the target model with a 22" barrel and the test gun in Hornady's book is a 19" barrel but also a semi auto. I did start back about 10% max. I actually haven't even check it for accuracy just made some for plinking on short notice. But I pulled out the chrono for a buddies gun and check these while it was set up and the results got me thinking. Thanks for the info.
Click to expand...

Your rifle has a 1 in 9 twist and may have a longer throat than a AR15 rifle which can effect velocity. My Savage bolt action .223 has a throat longer than my AR15 rifles and can be loaded "warmer" than the manuals. Listed throat lengths can vary from .025 to .106 and the AR15 has a throat length of .0500, and my throat length is .0566 for shooting longer bullets.

HOLLIGER ON .223/5.56 CHAMBERS
http://www.radomski.us/njhp/cart_tech.htm

What is the case capacity of your fired cases and what brand brass are you using. Case capacity can run from 28.0 to 30.6 and can cause chamber pressure to vary over 5,000 psi.
 
I ended up doing some load development for the gun for accuracy and turns out the original load was the most accurate. So I guess it might be slow but at least I put it where I want it. I haven't had anything to compare my chrony readings to. Maybe I should think about a new one on boxing day.
 
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