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Adding to what Jim has already posted:

Specifically in your ported rifle the gas checked bullets are necessary for more than just mediocre accuracy.  When a plain based bullet traverses the bore of these ported barrels, as the heel (base) of the plain base cast bullet passes the ports, those powder gasses are bled off incrementally by the series of ports, and in so doing those escaping gasses will erode the base of the bullet via a gas cutting action as the powder gasses are vented through the ports, escaping at right angles to the bore.   after passing several ports (as in the Marlin) this gas erosion can become severe, depending upon the intensity of the load.   Then, when the bullet finally makes it to the crown of the muzzle, there is no longer a square, sharp true seal at the muzzle when the bullet exits; instead there is an eroded base that allows powder gasses to escape first at the point of base erosion, thus causing the gasses to slip by one side or the other of the bullet thus tipping or cocking it as it leaves the muzzle.   In an unported barrel, the gasses remain behind the bullet until it clears the muzzle crown, thus pushing the bullet uniformly and squarely out the bore, because the bullet base has not been eroded as in the ported example described here.

Now, enter a gas checked bullet.  The guilding metal cup that is crimped into place on the bullet protects the base from the effects of those escaping powder gasses going though the ports on the barrel, and thereby eliminates any potential gas cutting and eroding of the bullet base.   When the bullet clears the muzzle, it does so point forward, square and true to the axis of the bore and without powder gasses slipping by the side of the bullet heel to cock or tip it as it leaves the bore.

Also, adding to the purpose of a gas check, in all barrels, not only does it help seal the bore, but the leading edge of the gas check is square and sharp, and acts as a scraper of sorts, cleaning some of the lead that may have been deposited on the bore by the bullet mass ahead of the gas check!  A neat plus of gas checked bullets.

Too, a gas check enhances the amount of torque that may be applied to a bullet without stripping the rifling.  It strengthens the amount of rotational force that a cast bullet will withstand, thus enhancing the accuracy of that bullet at higher than "normal" cast bullet velocities.

I'll stop... I could write a whole chapter for a book here!  This should cover the high spots of why a gas checked bullet in these ported barrels.

Hope it helps!

God Bless,

Marshall
 
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