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Posted: 3/17/2017 12:03:20 AM EDT
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So I've run into a bit of a conundrum.
I have an 11.5in barrel with a pinned 5.5in flash hider, I have a shooting range out to 100yds, and I'm looking to calculate bullet drop out to 300yds or so for an ACSS reticle. Without a chronograph, and without actually just going out and doing it. I'm trying to figure out the best way to do it. I don't know anyone with a chrono and can't cough up the cash for one myself. What I'm thinking so far is to research the fps lost per inch, find the fps of my cartridge, count down to my 11.5" from what the manufacturer measured it with, get the BC of the bullet I'm using, and punch it into a calculator. From there, figure out the drop that the ACSS is measured by, calculate how far each mark is, and then add up what each mark would be in yards for my cartridge. Would this work, and/or is there a better way? Approximately how close can I expect this to get me? |
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It doesn't matter what other people saw for velocity really. You'll really need to do your specific gun. Varies on ammo, ammo lots, how tight the bore is, etc.
You could try to work with 100yd and less to figure drop, but I don't think it'll be very precise at such short distances. If you could throw a scope on, Z at 100yd, then see what the drop is at 300yd, that would be great for taking the amount of drop to a calculator and figure out velocity. That's what I was forced to do when I started precision rifle, but I had a 300yd range. Barrel length will matter the most, what ammo you use would be a second, and probably how tight your bore is will be the third. On the bore thing, my LMT MWS has a stainless barrel with a tight bore. I get higher velocity out of that barrel than most people see. I also blow primers constantly with Hornady TAP though. (.308 win). This is why it'll vary from gun to gun so much. |
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What ammo do you intend on using, we should be able to get you in the ballpark. The how many fps per inch lost is usually not consistent I'm not looking to get dead on, I just figured it'd get close enough. Would the differences in velocity be putting me within a 8in or so, or would I be well outside that? |
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Quoted:
It doesn't matter what other people saw for velocity really. You'll really need to do your specific gun. Varies on ammo, ammo lots, how tight the bore is, etc. Quoted:
It doesn't matter what other people saw for velocity really. You'll really need to do your specific gun. Varies on ammo, ammo lots, how tight the bore is, etc. And thus my issue. I can find plenty of data for 10.5 and down to 7.5, but there's a gap on Google between 10.5 and 14.5 for me to get close to an estimate. Quoted:
If you could throw a scope on, Z at 100yd, then see what the drop is at 300yd, that would be great for taking the amount of drop to a calculator and figure out velocity. Quoted:
Barrel length will matter the most, what ammo you use would be a second, and probably how tight your bore is will be the third. |
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I have a sizable chunk of time to kill before work with not much better to do, so I went ahead and ran the numbers because I was curious.
11.5in barrel estimated numbers: 69grn 2631 fps 77grn 2506 fps Numbers were adjusted from the following Google data for IMI ammo (different sources), estimating a 30fps/inch change: 77grn 2740fps/24in -120fps = 2620fps/20in (24in is factory data) 2734fps/20in 2703fps/16in +120fps = 2823fps/20in 2866fps/20in Average - 2761fps/20in 69grn (could not find factory data) 2866fps/20in 2906fps/20in Average - 2886fps/20in I really couldn't find hardly any info or discussion on the 69grn, so I ended up with a much smaller amount to average from than I would have liked. I'm curious, does this seem even remotely close to 11.5in velocities? It seems a bit high to me. From what y'all are saying it looks like realistically I'll have to find a way to rent a chronograph, either at a range or for beer money and someone's time off a forum, but I'm just interested to see if punching it through formulas can get anything even remotely close to a proper number. |
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I own a chronograph and have recorded thousands of rounds that I have sent downrange. Razor 77 grain ammo comes from the factory HOT! Way hotter than anything I would shoot through my guns.
My standard match load for 69 grain SMK's is 24.5 grains of Varget in Winchester or Lake City brass. Remington 7.5 or CCI-BR4 primers. It goes 2850 fps from my 26" bolt action, 2750 fps from my 20" service rifle and only 2550 fps from my 16" carbine. Velocity loss in my testing has not been linear. I lose more velocity than expected when shooting ammo through shorter barrels. I suggest you zero at 200 yards. You should be around 3" high at 100. 10" +/- low at 300. |
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I own a chronograph and have recorded thousands of rounds that I have sent downrange. Razor 77 grain ammo comes from the factory HOT! Way hotter than anything I would shoot through my guns. I suggest you zero at 200 yards. You should be around 3" high at 100. 10" +/- low at 300. Quoted:
I own a chronograph and have recorded thousands of rounds that I have sent downrange. Razor 77 grain ammo comes from the factory HOT! Way hotter than anything I would shoot through my guns. I suggest you zero at 200 yards. You should be around 3" high at 100. 10" +/- low at 300. Would the +3/-10" be correct for an 11.5"? Quoted:
Using a 77gr SMK at 2300 - 2500fps, JBM says 5.2 - 6.5 MOA above a 100yd zero. Assuming POA/POI, aim at the top of a 6 MOA black circle and see where the bottom corresponds to the reticle. There's a SWAG. You'll have to excuse my noobness, but I'm unfamiliar with what JBM or SWAG are. |
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I ran the numbers on Hornady's ballistic program using 2150 fps for a 75 grain (.395 B.C.) bullets and got 3.1" high at 100, dead on zero at 200, 14.x" low at 300. Using a 200 yard zero you will never be over or under 3.1" all the way out to around 225 yards. In other words simply point and shoot in defensive situations.
I used 850 feet above sea level and 50% humidity when calculating these drops. You might get 2200 fps, maybe even 2250 fps with a 75/77 grain bullet out of a 11.5" barrel. Faster when using IMI Razor. IMI Razor has blown primers in some rifles and there are lots of internet talk saying so. That tells me that it's loaded way too hot and probably shouldn't be used. None of this means a damn thing. Until you actually do some hands on testing all we are talking about is theory. I zero every high power rifle I own (except lever actions and 7.62x39mm) at 200 yards. I understand I will be approximately 3" high at 100 when shooting .308's and I'll be 1.5" +/- high when shooting 5.56mm. I also know I'll be 2.5 moa low at 300 with the 5.56mm and 3.5 moa low at 300 with the .308. This is when I'm shooting full sized rifles, 20" AR-15's and 22" .308's. Every person who owns a firearm needs to learn their "come-ups", the number of clicks needed to shoot longer distances. Use a cheat sheet and tape it to your stock if need be. Finding a 300 yard range (or longer) to make this happen is not that hard. |
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Jbm is a ballistic calculator, app Swag is a some wild ass guess. Quoted:
Jbm is a ballistic calculator, app Swag is a some wild ass guess. Quoted:
I ran the numbers on Hornady's ballistic program using 2150 fps for a 75 grain (.395 B.C.) bullets and got 3.1" high at 100, dead on zero at 200, 14.x" low at 300. Using a 200 yard zero you will never be over or under 3.1" all the way out to around 225 yards. In other words simply point and shoot in defensive situations. Dead on at 200, 14 low at 300, but never over or under 3.1"? I'm a little confused. Point and shoot is what I'm looking for at defensive ranges, past that I'm just trying to find my velocity as near as I can, to get it into a calculator, find my drops, find the drops of whatever BDC reticle I end up with, a D then convert those drops to where I am. Quoted:None of this means a damn thing. Until you actually do some hands on testing all we are talking about is theory. Quoted:Every person who owns a firearm needs to learn their "come-ups", the number of clicks needed to shoot longer distances. Use a cheat sheet and tape it to your stock if need be. Finding a 300 yard range (or longer) to make this happen is not that hard. I really appreciate everyone's help, but I guess my little experiment doesn't actually work. Oh well, I've learned a ton though! |
| In the simplest of explanations, a ballistic program needs 1) muzzle velocity 2) specific bullet ( which equates to weight and ballistic coefficient). With those 2 things you can get pretty damn close out to 300 yds, now if you want perfect or are going for longer ranges you'll also need 1) temp 2) altitude 3) barometric pressure 4) wind speed and angle. It's really just a fun math problem. Oh and it's quite addicting! |
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Quoted:
In the simplest of explanations, a ballistic program needs 1) muzzle velocity 2) specific bullet ( which equates to weight and ballistic coefficient). With those 2 things you can get pretty damn close out to 300 yds, now if you want perfect or are going for longer ranges you'll also need 1) temp 2) altitude 3) barometric pressure 4) wind speed and angle. It's really just a fun math problem. Oh and it's quite addicting! It's that finding velocity part that's killing me. |
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That's actually super helpful, thank you.
From the information y'all have given me, punching in the 77grn (0.372bc) at 2400, 2450, and 2500fps, zeroing at about 0.5-0.7in high at 25yds would put me around dead on at 250, and a max +/- variation of 5.5-6.5in from 25 to 300yds. I'll use that as a starting point, and actually managed to find a private range with sub $200 memberships that's "only" 2.5hrs away from me. I'll be able to zero my rifle for half inch high at 25yds on private land where I usually shoot, and that should keep me on paper for finding the rest of my adjustments when I actually get time to drive all the way out to get a membership and range time. Thank you all very, very much, y'all have been a huge help. |
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Don't zero at 25yd like you mentioned. It's a more extreme trajectory than you'd want. At 100yd, you'll be like 8-10" high.
50yd Z should be pretty flat and inside the kill zone, but based off your real velocity for what bullet you end up using... Make it a point to travel to a longer range at least once? |
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Please read my post again. From up close and personal out to 225 yards you will never be more than 3.1" high or low when using a 200 yard zero from your SBR. After that things get dicey.
Over 14" in drop at 300 yards according to Hornady's ballistic program using the numbers I crunched. Approximately 4.75 moa, or 19 clicks on a 1/4 moa scope. SBR's are intended for close range home defense and house clearing raids by door kickers. They are useful and quick to action when working from a vehicle. They sacrifice a little ballistic performance to make them easier to handle. I would not worry about 300 yards other than knowing you are at your limit and act accordingly. Anything I need to do with a SBR is measured in feet, not yards. |
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