upper questions

Anyone have any experience with DSA ZM-4 uppers? They have a killer deal on them in the new SGN at $275. What is the reason or advantage of M-4 feed ramps? The other question that I'm sure has been beat to death is the reason for 1:9 vs 1:7 ? I would be putting this on an older (1980's) Olympic Arms lower that presently has a heavy bbl on it and is just getting to be heavier than I like to carry around. The other thing is that it will spend some time in a scabbard on the mule saddle, and I am no longer comfortable hauling my 1960's vintage colt ar-15 around in that manner,need more of a "throw away" in case of a wreck! We are doing a 2 week pack trip this summer too, and here again,the colt is way too valuble for this kind of use.

Mule,
I am not familiar with DSA upper you are looking into. However, I can shed a brief light on some of your questions...
The M4 feed ramps are designed for "more" reliable feeding of the ammo, than the traditional "old" style feed ramps.
You will have to ask yourself what size bullets you are shooting, to answer your 1:9 or 1:7. Are you looking into 62grain or 55 grain or ??? size bullets. Some claim that it won't make that much of a difference..... but if you plan on distance shots, a properly stabilzed bullet is the ticket.
I am sure a boatload of experts will weigh in soon! ;)

I'm not planning any long shots with this one,but there is no point in building something that isnt capable of good long shots,if it works good way out,it will still work good up close,right? It cant be scoped and still fit in the scabbard,but here again it may not always be used for this. Sowhich twist provides better stabilization?

Mule,
I believe conventional wisdom says 1:9 is a good all around twist. I think one expert told me that it will stabilize from 55grain up to 69grain. But not ideal for the end weights of the spectrum.

I do believe my Oly 20 is a 1:9 and I shoot all sorts of weight out of it.

The 1:7 twist seems to like higher weight bullets. I think wisdom says bullet weights in the 70+ grain area. However, I have references that say that 1:7 can easily handle down to 55grain.

What length barrel are you thinking of?

When you ask a barrel twist question, you don't always get straight answers!
Hopefully we can find some experts with actual 1:7 rifling to answer you directly!


I think the pearl of wisdom is that the 1:12 is for real low weight bullets.
I have included a link to a website that discusses the .223 round... hope this helps.
bcart

The 1/7 twist is mostly for the long military bullets. The tracer is almost 1 1/4 inches long. The ball is about an inch and m193 is about 3/4 inches. The longer the bullet the tighter you need. The civilian bullets don't have the metal penetrator so they're not as long. Therefore a 9" twist will likely fulfill all your needs for softpoint and civilian bullets. By the way, no hollow points please. I don't think anyone has very good feed with them.

I think 1 - 9 is what it needs to be. 16" bbl,needs to be short and light for horseback use.Also flat top.I have a couple 5 rnd mags ,they are pretty flush so as to make it easy to draw. The easy part is setting up the rifle,the hard part was getting the mule to let me shoot from here back!

Well,it came yesterday. It looks ok,nice fit and finnish,my bolt and carrier works smoothly in it. The only problem is where the tang fits into the lower at the rear.The tang is 0.011" bigger than the slot in the lower so some carefull file work will be reqd. It may be that the oly slot is out of spec too,so I will check to see how my other upper fits in it before working on the upper.

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Originally Posted by mule

What is the reason or advantage of M-4 feed ramps? The other question that I'm sure has been beat to death is the reason for 1:9 vs 1:7 ? I would be putting this on an older (1980's) Olympic Arms lower...

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If the upper has the M4 feed ramps make sure your new barrel will also have them! Otherwise, feed malfunctions are MUCh more like likely.


If you can get a better barrel profile for your needs by going to a 1-7 twist, don't let it put you off. With decent bullets/ammo you'll not notice an accuracy difference, even with lighter bullets. See link below for a brief test using both short (light) and "heavy"long, non-match bullets- which are the ones most likely to give poor results:

M193 and M855 out of a 1-7" twist barrel

'80s Olympic receivers and such are often a bit out of spec. If you can make it work, it ought not be a worry unless you change uppers frequently. Then, you might consider correcting the lower rather than the upper. (I finally gave mine to a sister as a lightweight carbine- she won't change a thing, and it runs great as is.)

After thinking about it more,I'm going to take my other rifles apart and see just what is out of spec. If I need to open up the oly in the mill,then thats what I'm going to do. I dont want a bastard rifle.

Mule,
I have been out of pocket for some time.... how is the rifle/ combination coming along? What did you find was out of tolerance?
And of course we need a range report! ;)

It was the lower. I changed the upper just because it was far easier. Fits nice and snug,pins are too tight to come out by hand,but a cartridge will do it. Ive been working the bolt with the handle,cycled it 400 times or more,so its ready for the first shot Ive got a lot comming up for the w/e,but I will see what I can do. At least I have my own range!