no the one in 9" is not better --a 1-7" will shoot all the heavys down to 55gr bullets , a 1-9" generally does from 75gr loads down to 50gr or less --like 40gr varmit loads. the one in 9" barrels are really 1-8.75" anyway--
with most common bullet lengths they act about the same --some 1-7" barrels will shoot a point of aim with 55gr bullets that is not same as they shoot with 63,69 and 75 grain loads. i have found the 1-7" barrels to be just as accurate with the 55 as a 1-9".
there are lots of things that affect accuracy -- i suspect that many of the things we hear are more than likely ammo issues rather than which twist rate is best.
i consider a 1-9" and an 1-7" both very good many 1-9" barrels will shoot 75gr loads with ease. personally i think the 1-9" vs 1-7" thing is a non issue. just get whichever one you like and shoot anything between 55gr and 75s--
the 1-7" is a little better in real shot barrels like 10"--shooting 77 grain loads, the issue is the length of the bullet the longer the bullet the more twist it needs.
most loads work very well from a 16" in 1-9". a general rule is the longer the barrel the less critical the ROT is. i know many that shoot heavy loads from 1-9" barrels. i frequently shoot 75 gr loads from a 20" 1-9" it does very well.
there is a lot of overlap in the 1-7", 1-8" and 1-9" barrels with what they will shoot. generally any of them are good for most loads from what i have seen. when it gets more iffy is running really long bullets from a really short barrel , or running very light short bullets from a barrels with a very tight ROT like a 1-7"
the most often used bullet weights 55gr-75gr run good in all of um.
Just checked wth MSA...6 months wait...will look at other options...
Another question: Does the front sight base interfere with lower mounted optics? Wondering if I am better off with front gas block sight base..
over at those mall-ninja ar-15 places they argue for days about the 1-7" vs 1-9" , the mill-spec mall ninjas got to have a 1-7" the el-cheapos want the 1-9" to save a few bucks.
i have had both for years -- probably the best all around ROT would be a 1-8" that is usually run in target type barrels--LOL!
best i can tell the 1-9" 1-7" can both shoot 55gr and 63 grain stuff pretty well . some barrels shoot better than others in the same ROT --
what really matters is how good a given barrel is --whatever number of twist rate is just more or less icing on the cake between a 1-7" and 1-9"
nothing wrong with the SA m4 that i am aware of. --it is quite a bit more expensive than that DSA !
but it will run just like any other will with a good BCG , charging handle and so on. those hand-guards ain't expensive CDNN sell the doubl;e lined m4 units for $10--
Try Sota Arms or RedX arms, bought from both and they are pretty good, don't have a link just google . I think they are getting caught up some now