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300 blk super/sub from 7.5" barrel - bullet drop charts in 1in increments? Login/Join 
Rock Paper
Scissors
Lizard Spock
Picture of James in Denver
posted
I've just built a 7.5" 300 BLK upper (and lower combo actually). While 7.5" is not ideal, it's what I had.

Anyway, I have done a fair bit of google-ing and I can't find a drop chart for 220/200/125 grain 300 BLK ammo out of a 7.5" barrel.

I have a chart for a 16" barrel comparison (comparing to 556) and another set of stats showing differences to POI with a 9 inch barrel at 100/200/300 yards, but nothing with 7.5" and nothing showing the various zeros.

My wish is to zero the "pistol" at 25/100 (or whatever 2 points, 25/?) for the SUBSONIC since that's the main load I'll be shooting from it.

I want to then know what that's going to do to the 125 supers, where's the hold-overs (hold under?).

I don't have any ballistics software, and even if I did, I don't know what to enter (bullet coefficient, etc.) to get what I want.

The rounds I'll be using to start are the Remington OTM 125 and OTM 220s, got a few boxes before Wallyworld stop selling "dangerous" ammo.

Oh, and I live in the Denver metro area, if that matters.

I'm using HK sights, which are slightly lower than standard AR sights, I think they are like 2.25 over bore or something rather than 2.6 (HKs are supposedly 3/8s lower)?

James


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Posts: 4476 | Location: Colorado | Registered: August 24, 2009Reply With QuoteReport This Post
Freethinker
Picture of sigfreund
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I’m sure all that would be easy for some of us to calculate, but the two musts for inputs are sight height over bore and muzzle velocities of the loads you’re interested in. Sight height is something you can determine simply by measuring the distance as closely as possible between the top of the front sight post and the center of the bore.

If you can’t measure the velocities yourself, an Internet search might turn them up. (A quick and limited search myself didn’t reveal any specific data, but there is some YouTube stuff that might allow reasonable extrapolation. Something to consider.)




“A man hears what he wants to hear and disregards the rest.”
— Simon & Garfunkel, The Boxer, 1970
 
Posts: 41413 | Location: 10,150 Feet Above Sea Level in Colorado | Registered: April 04, 2002Reply With QuoteReport This Post
Member
posted Hide Post
sigfreund is correct that POI comparisons will be guesses without accurate sight height and MVs.

Lacking accurate MV, one can back into the figure using extremely precise impact data over probably a handful of distances -- with longer distance data being more important than shorter distance data.

If you have good MV data for 125 ammo in a 9" barrel, expect roughly a 100 fps loss of MV for a 7.5". But it could either more or less. Ballpark MV might be 1800 to 1900 fps. Probably more towards the lower end.

You won't see much difference between 9" and 7.5" barrels for the subsonic 220 load. Expect an MV somewhere between 1000 and 1050 fps.

At 25 yards the 125 ammo will likely impact a little higher than the 220 ammo. Maybe 1" to 1.5" higher.

At 100 yards the 125 ammo will likely impact 7-11 inches higher than the 220 ammo. Subsonic 300blk drops like a stone.
 
Posts: 6429 | Location: Colorado | Registered: January 26, 2008Reply With QuoteReport This Post
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