Good Morning Oldarmy,
Yes, a .400"size ball would improve accuracy in a GM 40 caliber barrel.
In my GM 50 caliber barrel, I use 0.018" - 0.020" thick patch material. Both work equally well and give match grade accuracy.
If you will closely examined the muzzle of a GM barrel, it can be seen that that the rifling groves are considerable wider than the lands.
It has been my experience over the years that this type of rifling style will almost always produce the best accuracy with a bore size ball and is not that difficult to load with a tight patch/ball combination.
Some of my other barrels that have that type of widegrooves/narrow lands rifling also produce the best accuracy with bore size balls.
One barrel in particular, has very wide grooves and produces exceptional accuracy with a ball cast from a .500" size mold, that actually measures .5035" in diameter.
A new benchrest barrel is on the way and has been cut with very wide grooves. It is anticipated that the .5035" size ball will produce the best accuracy.
All of my older Douglas barrels produced the best accuracy with bore size balls. The old H&H barrels by Hoppy Hopkins had the wide groove groove/narrow lands style of rifling and had a decided preference for bore size balls.
All of this is right in line with what my shooting friends have experienced and still experience.
However, in the end ,as always, only experimenting with patch and ball combinations will determine what is best for any individual barrel.
Best regards and good shooting,
John L. Hinnant
If you are not an NRA or NMLRA Member, why not? I am carrying your load.
Politicians Lie!! US Freedoms Die!!!