Any bending of an SKS barrel by a bipod is minute in gross mechanical terms. Unless the rifle is severly abused, or the bipod weighs fifty pounds, I'd say that's a myth.
Adding a bipod or any barrel-mounted accesory can "bend" the barrel affecting accuracy, or change the point of impact in comparison to the point of aim. However, it's not "bending" in the sense as in you hit it with a sledgehammer.
Every barrel bends and flexes somewhat when the gun is fired. If you've ever taken a long thin plastic pipe and shaken it at one end and you see how it wiggles, a gun barrel does the same thing but to a much smaller degree, but it's enough to noticably change where your bullets go at a hundred yards or more. All rifle barrels flex like this, unless the barrel is insanely heavy and thick, or short in relation to it's thickness. Some rifle designs flex when fired more than others, all depending on the barrel length, the caliber, powder charge, and velocity of the shot, the thickness of the barrel steel, and how it's mounted to the reciever and the stock.
This flexing isn't a problem as long as it's consistent from shot to shot. When it's consistent, (Negating wind, bullet & load variances, and the fidgeting of the shooter) the bullets are being fired at the same trajectory each time, and the sights can be adjusted to line up with the point of impact.When people say "accuracy" they really mean two things. accuracy AND consistency. Accuracy is the rifle hitting where it is aimed. Consistency is all the bullets hitting as close together as possible. A rifle could be extremely consistent, putting all the bullets in almost the same hole at 100 yards, but if it's hitting 12 feet to the left of where it's aimed, it could also be wildly innacurate. A good rifle needs both.
Bipods can affect accuracy or consistency when they're mounted to the barrel because it adds a completey new factor into the rifle's "natural fequency" of flex when it's fired. The bipod can be open, or folded, the rifle's weight (and a bit of the shooter's) can be resting on the bipod, or the weight of the bipod can be hanging from the barrel if the shooter is shooting from a standing position instead. All introduce random factors into the barrel harmonics when fired, and can cause a difference in the point of impact each time.
When you see rifles such as target rifles, high end hunting/varmint rigs, or sniper rifles, equipped with bipods, you'll notice that the bipod is ALLWAYS mounted to the forestock instead of the barrel. And the stock is only attached to the the rifle at the rear on the reciever, and the barrel itself is not supported by the stock in any way. This minimizes any external pressure, weight, or influences on the barrel allowing it to flex consistently each time it's fired.
Since the bipod isn't bending your barrel in the gross sense, the grenade sight acting as a structural brace is irrelavant. Instead it’s one more barrel attachment that’s affecting it's flex in it's on independent way. The SKS is full of barrel attachments that can affect it's flex as it is, the bayonet & bayonet lug, the cleaning rod, the grenade launcher muzzle attachment, and the gas piston are all pushing and pulling on the barrel when it's fired. This can make things very dynamic as compared to a bolt-action rifle with an unencumbred free-floated barrel.
That, and other issues with the SKS's manufacturing tolerances, and the quality of common 7.62x39 ammunition mean that the SKS would not normaly ever achieve a level of accuracy that's 1 minute of angle or less. And, it's spread might actualy be many times that. However, it's not really a design flaw, as that's not what the SKS was for. It's a medium range infantry carbine that's meant to hit man-sized targets at 300 meters or less (Despite how high the sights go… Most modern infantry engagments are 100 meters or less.) and do it in less than ideal battlefield environements. That the SKS can do admirably well.
So by adding your bipod, you may see significant changes to your accuracy and point of impact in relation to your sights, and if you are changing shooting positions while the bipod is attached you may lose consistency in how tightly it groups as well. However, that may all be evened out by the added stability to your firing positon that the bipod gives. You just have to test if adding a bipod increases accuracy more by giving you stability in your hold on the rifle than it takes away by changing the barrel flex.
