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I'm thinking about building a 12.5" 6.8 SBR. I don't reload due to family and space constraints (no space for reloading bench), so I'm planning to shoot factory ammo for the foreseeable future.

.300 Blackout is an option, but while this rifle will be suppressed quietness is not my primary goal and a pistol load in a rifle just seems silly to me.... :p

Is there an online ballistic calculator that I can use to compare the performance of 12.5" vs 16" vs 18" rifles using factory loads? Most of the resources I've found require user supplied ballistic coefficients and velocities. Without a reloading book I don't have this info readily available.

(I searched the stickies but didn't find anything helpful)


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I'm thinking about building a 12.5" 6.8 SBR. I don't reload due to family and space constraints (no space for reloading bench), so I'm planning to shoot factory ammo for the foreseeable future.

.300 Blackout is an option, but while this rifle will be suppressed quietness is not my primary goal and a pistol load in a rifle just seems silly to me.... :p

Is there an online ballistic calculator that I can use to compare the performance of 12.5" vs 16" vs 18" rifles using factory loads? Most of the resources I've found require user supplied ballistic coefficients and velocities. Without a reloading book I don't have this info readily available.

(I searched the stickies but didn't find anything helpful)

Sent from my iPhone using Tapatalk.
For the most part you can find ballistic coefficient info on the ammo and/or bullet manufacturers website. You will also find velocity numbers on their websites also...for the most part rifle ammo velocity figures are based on a 24" test barrel (with some exceptions...like Federal Fusion MSR and Hornady 6.8 SPC 110 BTHP American Gunner ammo for example). I'm not sure exactly what data you are looking to compare but Hornady's site lists all the info you need for all of their ammo so you can plug in some numbers and get some fairly reliable estimates. Generally speaking you can subtract/add 25 fps per each inch shorter/longer than whatever the test barrel length is to get a ballpark figure of what the velocity should be for different barrel lengths...I have found that method to be reliable enough for estimating velocity...however, I have only done that for barrels as short as 16"...I do not know if the 25 fps estimate still applies as the barrel gets increasingly shorter than 16".

There are plenty of ballistics calculators available online but my preferred one is: http://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi
 

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Muzzle velocity is one of the key inputs into a ballistics calculator although some are preloaded with muzzle velocity data published by the manufacturer, but then you have to figure out if that muzzle velocity is applicable for your rifle as the manufacturer likely used a different barrel length.

I don't believe any ballistics calculator is going to adjust muzzle velocity based on barrel length so you are going to have to make some assumptions or just find something written by someone who has already studied this before.
 
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