Barrel Deflection

Sometimes, one hears the term "barrel deflection" when speaking of rifle shooting. Simply put, the term refers to the effect had on a barrel from external pressure bending a the barrel a microscopic amount. With the AR15, there are a multitude of tubes, rails, and other free float handguards for the consumer, but they all basically work in the same way: the device isolates the barrel and has no separate point of contact other than at the receiver.

Since there are two broad ways to assemble the barrel to the rifle: free-floated and non-free floated; there are naturally arguments about the pros and cons to each concept. Broadly speaking, free-floated rails are inherently more accurate. Sometimes (not always), the free-floating device adds weight to the rifle. The arguments surrounding non-free floating revolve around A) reduced cost, and; B) only marginal increase in accuracy for free-floated devices in controlled environments.

However, almost unique to the AR15, you will occasionally see internet commandos spout the fact that the accuracy increase is only marginal. For example: a given non-free floated barrel might be capable of 2 MOA accuracy; yet only increase to 1.0 - 1.5 MOA accuracy after free floating. People who prefer the non-free floated setups (often the more "budget-conscious" consumer), spout this fact as their slogan of choice.

These statements are misleading, as slogans always are.

Take this experiment for example:


Fixed Variables:
Rifle: AR15 - 20" barrel - non-free floated.
Ammo: 55-grain bulk ammo (Monarch brass cased)
4-shot groups.
Distance: 25-yards

Experiment 1:
Rested on the forearm vs. magazine rested
The top group was shot first, resting the rifle comfortably on a field rucksack.
The middle group was shot resting on the magazine as a monopod.
Clearly, the first (top) group was 2" high.

Experiment 2:
Group 1 - Rested on a rucksack
Group 2 - Rested on magazine as a monopod
Group 3 - Tight sling used. 

Clearly, the two-inch (8 MOA) deflection experienced when propping on a rucksack rest is repeated.
Interestingly, the deflection from magazine rest to sling rest is only a quarter-inch down (1 MOA), but is an inch to the left (4MOA). 

Comment - comparing resting on a rucksack to sling use, and there is 9 MOA of shift vertically and 4 MOA horizontally.  Invoking use of the pythagorean theorem, the direct MOA shift is 9.87.

Results
A near 10 MOA shift difference experienced from ruck rested to sling use makes long-range shooting over 200 yards sketchy, if not downright impossible.

Also note that with the exception of a single flier in group 1 of the second picture, the rested rifle groups were very tight, whereas the sling use opened the group up consistently. This shooter has shot consistent sub-1" groups at 25-yards with a free-floated AR15. A near 2" group like this is unacceptably errant.