Rifle Balance calculator

Want to adjust the balance of your rifle, but don’t know what length of barrel to order? Lets see if we can figure it out.

Recently MDT shared a video showing velocity changes as a .22LR barrel was cut down from ~46 inches to ~12 inches.

Since we get constantly asked questions about the barrel length for .22 trainers I felt it was worth summarizing their findings and posting a short blog post to track the information, and talk about another common question we get which has to do with rifle balance.

This rifle balance calculator is a work in progress – BETA mode. I am trying to figure this out as well, so if you run the calculations and arrive a different conclusions, please let me know. If it works for you also, please let me know.

Summary of the MDT .22LR barrel length video

Around 16 Inches had the most velocity (~1080 fps), and after 16 inches velocity decreased to ~1055 fps at 23 inches. The maximum loss of velocity between 16 inches and 26 inches (the max range most people consider for barrels) was 25 fps during this test.

Barrel weight of a straight contour barrel is ~10 pounds for a 28″ long 1.25″ diameter blank. So .34 lbs / inch. So the variation in weight for a barrel of 16 inches long to 26 inches long is approximately 3.45 pounds.

Many people consider a lot of different variables when trying to decide the best barrel length for their rifle.

• Velocity impact on the projectile (more or less velocity)
• Overall Rifle weight
• Balance point for the rifle

All this talk lead to the most common question I get which is how a particular barrel will affect the balance of the rifle.

Calculating Balance point using a Center of Gravity calculator

Lets look at my RimX 22LR Barrel with action in a KRG Bravo stock with a RimX Prefit Barrel (Bartlein 20″ Heavy Palma).

• Rifle Length (total) = 38.25 inches
• Rifle weight = 12.5 pounds
• Barrel length = 20 inches (Heavy Palma Contour)
• Barrel Weight = 3.8 pounds

First I want to calculate my current balance point using a formula (or this online calculator).

Document the following:

• COM = balance point of rifle with barrel attached. (to check our work later)

Then remove the barrel from the rifle – leaving all the other parts attached (magazine, bolt scope etc). And document the following:

• M1 = Weight of Rifle w/o barrel
• M2 = Weight of barrel
• X1 = balance point of rifle w/o barrel measured from buttstock of rifle
• X2 = balance point of barrel when attached to the rifle
  • The above is going to be a calculation achieved by measuring the balance point of the barrel while detached from the rifle. Determine this, and mark the barrel with a sharpie or pencil. Then to get X2 re-attach your barrel to the rifle and measure from the buttstock to your mark to get the correct number.

For my rifle I got the following measurements

• M1 = 8.7 lbs
• X1 = 16.5″ from buttstock
• M2 = 3.8 lbs
• X2 = 24.5 ” from buttstock

The actual formula is [ Center of mass = (m₁r₁ + m₂r₂ + … + m_Nr_N) / (m₁ + m₂ + … + m_N) ] where

N – number of masses m (we are using 2 masses)

r_i – distance from the reference point (in our case the buttstock)

Using the above formula for calculating the center of gravity, we can calculate where the current balance point should be, then verify.

Using our numbers, the calculator suggest my balance point is 18.93″ from the buttstock. I measure my balance point at 19.5″ so the numbers are off by .57″ (calculator is under by .57″).

Measured balance point is .57″ towards the muzzle from the calculated balance point. (long)

Why is my number wrong?

There are some reasons why our calculation is incorrect even though we followed the above calculations correctly.

• It is difficult to measure some of these accurately, so there is some fudge factor
• the primary reason I see is that the calculator assumes the masses are densely located along the X coordinate. For example 100% of the M1 mass is at X1 location.
• In our case the masses are spread out in weight across the X coordinate, and thus the weight is some percentage of the measured weight.
• In our real world rifle, the pivot point doesn’t separate the masses, some of the mass crosses over the pivot point and is positive and some is negative. The barrel may extend back past the pivot and the chassis may extend forward of the pivot.

Though my measure number is close my sample size is small and you may experience differences.

Testing a change (move the balance point)

I have a heavier contour (but shorter) barrel on hand. Lets see how the calculator work for this example.

Lets try a Bartlein MTU 17.5″ barrel.

The only numbers that change are M2 & X2.

• M2 = 4.9 lbs
• X2 = 26.63″ (measured)
  • balance point of barrel off rifle = 8.63″ from breech face
  • 8.63 – 1.75 (tenon length) = 6.88 (balance point from shoulder)
  • 6.88″ + 20″ (length of buttstock to face of action) = 26.88″
  • my calculated # and my measures # are a bit different.

My calculated balance point would be FORWARD to 20.15″ moving the balance point forward 1.22″ according to the calculator.

When I attach the barrel, I measured my X2 at 26.63 which is shorter than my calculated value of 26.88. And my measure balance point is at 19.75 which is .4″ shorter than the calculated value.

Measured balance point is .4″ towards the buttstock from the calculated balance point. (short).

However the balance point moved .25″ toward the muzzle from this change in barrel.

Testing Summary

While the calculator was not exact, in general it helped determine the direction a change would be. The calculator indicated the balance point would move towards the muzzle with the projected change, and indeed it did move that direction.

Our test was with a fairly small change, and was a very limited data set. I will test with more chassis and configurations as I have time.

Details

Below is the graph from MDT about the velocity drop. Click this link to watch the entire video about the changes in velocity with barrel length for .22LR.