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Help – Suppressor’s Impact on Trajectory

PostPosted: Fri, 23 Nov 2018 20:45:53
by MarcSpaz
My apologies in advanced for a long post.

I am experiencing an anomaly, at least anecdotally, with a specific weapon system. I am hoping someone may be able to help me understand what is happening.

I have a Remington Precision Rifle in 6.5 Creedmoor. I have been using Hornady 143 grain ELD-X and 140 grain ELD. I mounted a Burris XTR II 3-15x50 scope (MRADs) on it. I also have an AAC 762SDN-6 suppressor that I use with it.

I understand that the suppressor will impact barrel harmonics. The suppressor typically slows the round down (measured 100 FPS slower at the muzzle with a chronograph). I also understand it can cause projectiles that are prone to yaw to reach their peak angel sooner, affecting flight path and drag CoE.

I zeroed the rifle system at 100 yards with no suppressor and set the zero-stop. My 200 and 300 yard calculated adjustments are spot on based on the ballistic calculator. Today, at 200 yards, I came up 0.3 mils from my 100 yard zero, and at 300 yards I had to come up 1 full mil.

Now… here is the part I don’t get. I thread on the suppressor and my point of impact changes, causing me to have to add an additional 1.4 mils to all of my adjustments. So, the shift looks like this:

100 yards
Zero changed to 1.4

200 yards
0.3 changed to 1.7

300 yard
1 changed to 2.4

These results don’t even remotely match the 100 FPS speed decrease when it comes to trajectory calculations. Prior to getting the RPR, I have run this exact can on a wide range of firearms, with barrel lengths varying from 9 inches to 18 inches. I have used it with .223 Rem, 5.56 NATO, 300 Blackout, 7.62 NATO. From point-blank to 300 yards, no other weapon I have used the suppressor on has shifted more than 0.1 Mil. Based on the ballistic calculator, the speed decrease calls for an additional 0.1 mil come up… which is what I expected.

Has anyone experienced this before? Is this due to a change in barrel harmonics? Or is there something new I need to learn about?

I would like to continue to use the suppressor, but I don’t know what to do as far as ballistic calculations. Just broadly adding 1.5 MRAD to each dial doesn’t sound like a safe method to try, even though that has been the case for the first 300 yards.

Besides for getting out there and shooting (building a DOPE card), do you guys have any recommendations on what I can do to discover the source of the deviation and compute for it?

Re: Help – Suppressor’s Impact on Trajectory

PostPosted: Sat, 24 Nov 2018 10:41:01
by jdonovan
speed change is just one part of the physics going on.

Think of the harmonic of a barrel looking like a sine wave.

If the bullet exits near the minimum or maximum then there is a comparatively long time where the barrel moves very little. If the bullet exits near the zero of the sine wave then the barrel is moving quite a lot.

Depending on the weight of the can, the length/stiffness of the barrel, the weight/speed of the projectile the addition of a supressor can have very large, or very small differences. Each case is going to be a unique case.

Also without an index mark, you're likely to thread the can on to slightly different position each time. Which may lead to slightly different shifts up/down left/right. Over short distances probably not a bit deal, but over longer distances a 0.5 MIL shift can be significant. Especially when it is an inconsistent shift.

a dope chart is critical for long range work, but its only good if the dope is repeatable.

Re: Help – Suppressor’s Impact on Trajectory

PostPosted: Sat, 24 Nov 2018 13:41:21
by MarcSpaz
That makes a lot of sense. I almost think that I may not be able to use the can for long distance if that is the case. The mount style being a ratcheting quick connect actually leaves a little play between the suppressor and the mount. The can moves around a bit until the system heats up. I doubt it locks into the same position once it finally settles.

Thanks for the feedback.