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Hi folks,
I decided to get some figures on this rail flex issue for free-float systems. I've only got a Mk12 Mod 1 SPR clone on hand to test so I used that as my testbed. It sports the correct KAC rifle-length free float system and is not a particularly heavy or light example for a 12" rail.
I set up a dial indicator (Compac 213GLA .0005" indicator and later Interapid 312B-3 .0001" to compare readings) on a magnetic indicator (Brown & Sharpe) base with a vee-groove. The rail is aluminum so I attached the base to the barrel, centering it with the vee-groove and butting it up against the thread protector on the Ops Inc muzzle brake so it won't move around. The indicator is at the 12 o'clock position. Without setting the gun on anything to prevent any and all possible deflection, I zero the indicator on a flat spot on the front BUIS. I did this with the gun's buttstock on the ground, the firearm vertically oriented. This allows me to measure the total movement between the barrel and rail, but requires me to differentiate if the barrel itself or the rail is moving.
For reference with these small numbers, a sheet of printer paper is .004" thick.
The first thing I noticed is the barrel sags from gravity alone. It is noteworthy that the SPR barrel is 18" long and utilizes a heavier contour than seen on a typical carbine. With the dial indicator zeroed to the rail, there was an immediate .0075" reading when I changed the orientation of the rifle from perpendicular to parallel with the ground. The rifle is being supported by the receiver with both hands. I am not supporting it at the rail with my support hand at all which might deflect the rail and invalidate the figure. To make sure this wasn't the base coming loose, I held the rifle vertical again and the indicator returned to zero. Then I held it upside down and the dial indicator read .0055" the other direction. This means the barrel is sagging from it's own weight. There is a discrepancy of about .002" between the amount the barrel sags depending if I hold it normally or inverted. I am guesing this is because the gas tube might lend stiffness to the barrel when it's in compression (when the rifle is inverted upside down) versus when it's in tension (held in a normal position brought to bear).
Next, I put my support hand under the rail as if I was shooting offhand without a sling using a Magpul AFG, pulling the rifle against the pocket of my shoulder. The indicator reading increases .0025" to read a total indicator reading of of .010". That is, the initial .0075" from the barrel sagging plus .0025" of rail deflection holding the rifle off-hand.
Next, I attached a sling and cinched it so my support hand was pushing against the AFG to get tension on the sling. I get a .010" total indicator reading, nothing different. I reoriented the indicator to the 9 o'clock position to see how much the rail is deflected away from the barrel with the sling pulling on it. I zeroed the indicator with the rifle in the vertical orientation. Bringing the rifle back to horizontal did not change the reading because gravity wasn't pulling the barrel to the side. With the sling tensioned up, I managed to deflect the rail itself .005-.010" horizontally, which varied depending on how I positioned the elbow on my support hand wrapped in the sling.
Next, I wanted to use the bipod. I am using the Harris bipod located at the very end of the rail. When I set the rifle down on a hard table with the bipod deployed so the rifle free-stands on the bipod and buttstock, the indicator reading grows to .013". Subtracting the .0075" sag of the barrel, we can see that the rifle resting on the bipod deflects the rail .0065" with only a BUIS on the end of the 12" rail. If you hang lights, lasers, grenade launchers, NVS, this will undoubtedly increase the deflection on the rail itself if it rests on a bipod.
Out of curiosity, I wanted to see how weight affected the barrel. I didn't have room to fully thread on my 12th Model because the indicator was affixed to the barrel, but I did just slip it over the muzzle so the can hung off the muzzle brake. The barrel deflected the indicator .0025" due to the additional weight hanging off the barrel. The barrel itself appears to sag .010" overall with the Ops Inc 12th Model can, which is all stainless and pretty hefty.
With the can removed, once I lift the buttstock off the table, the reading increases from .013" to .015" because the center of gravity changed and there is a weight shift towards the bipod. I did notice when I went prone and got behind the gun, the reading did not change any, even if I preloaded the bipod with some force and tried various ways of supporting the buttstock with my support hand.
It seems .010" of horizontal deflection can be created by sling use for my particular example and about .0075" vertical deflection of the rail. I would not consider the initial .0075" of sag from the barrel as part of the equation for overall deflection because you'd zero the rifle with the barrel horizontally, not pointing at the Sun. Rail deflection appears to be worst with sling use, followed by bipod use, and lastly off-hand use. A rifle that uses a bipod like the Mk12 Mod1 will be zeroed with the bipod deployed and fired with the bipod most of the time. I've never actually fired my Mk12 off-hand because it's a hefty beast so I don't know what, if any POI changes there would be.
Theoretically a vertical deflection on the rail of .0075" over an 18" long barrel would equate to .005" of divergence per foot traveled, or 1.5" at 100 yards. A maximum horizontal deflection of .010" from sling use should mean a divergence of .0075"/foot or 2.25" at 100 yards. I don't know if this translates because I've never attempted to document POI shifts with various shooting positions. Anyone care to chime in?
Last edited by Cesiumsponge; 02-28-11 at 21:32.
“The practical success of an idea, irrespective of its inherent merit, is dependent on the attitude of the contemporaries." Nikola Tesla
Good post. It would be interesting to see how these would compare against a Service Rifle with a bull barrel and a steel free float tube. Assuming it too would deflect, the main thing is that the deflection is consistent from session to session and that the load is tailored to the barrel whip characteristics of the barrel (i.e., bullet exiting at a "full up" or "full down" node).
Your math is correct. The mistake was giving your answer as foot pounds instead of inch pounds. For 20 lbs of weight to exert 200 ft/lbs, you'd need an arm of 10 feet
I should have shown my math:
20 lbs x 10 inches = 200 in/lbs.
To convert in/lbs to ft/lbs, divide in/lbs by 12. 200 divided by 12 = 16.666 or 16.7
So does this equate to loss of accuracy during rapid fire, that should be a concern, or metal fatigue of the upper itself? The DD vid showed a substantial amount of flex in the barrel during firing, enough that even I would be concerned about fatigue & or accuracy. I have a DDL 14" that I bought so I could extend my arm out as far as possible & to extend the sight picture. I trust the way that it's mounted & how it cinches down. My barrel is a BHW 16" M4 profile & the upper is a BCM. All quality parts that I have confidence in & I'm not all that worried about these parts failing any time soon. With all of the guys running 3 gun & tactical courses, if it was a concern in terms of accuracy & failure, I think one of the guys here would've seen any form of fatigue or failure at some point. With all the different combination's & configurations, it would take years to gather enough info to make solid data available. So, what's up Rob? Why's it a concern?
That's a great question, and kind of the reason I started this thread to begin with.
We've gotten a lot of discussion on what flex is, what deflection is, what displacement is, but still not a lot on whether they matter, or whether one matters more than another.
I think we may be headed down the rabbit hole (or circling the drain?) without any end or purpose in sight.
Does flex (see definition below) matter? If so, what is the negative effect in the short term single shot), mid-term (multiple shots) and long-term (life of the rifle)? Are the effects of flex the same or different for barrels vs. rails?
I think it's fair to say that displacement, flex without the return to center, is a problem in both systems.
Last edited by rob_s; 03-01-11 at 08:35.
In that thread I used the term flex/shift which isn't the correct term.
The problem I saw with 2 Samson rails was that the rail fit too lose on the 2 inserts and fit too lose on the bbl nut. This allowed the rail to shift around on the barrel nut and the rail would stay in that position. Not very good just as lose lug nuts on your car isn't good.
Chief Armorer for Elite Shooting Sports in Manassas VA
Chief Armorer for Corp Arms (FFL 07-08/SOT 02)
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