Greetings,

This may have been covered before but please bear with me. I just assembled a new M4. It is my first build.  All the parts are brand new and the upper and bolt are from the same manufacture.  So far I put 400 rounds through it with just one reoccurring hiccup. That is that when I insert a fresh magazine and feed the first round, the bolt wouldn't slam home, I have to push the forward assist several times before the bolt is seated. After the first round is seated, the rest of the magazine runs fine. Any ideas?

If this makes no sense, forgive me, the New Years celebration has begun and I am just trying to lessen another worry in 2006

How many rounds are you stuffing into the mag??

FWIW I ,as a rule, only put 28 rounds in my mags.

does this happen across several known good magazines??


tried different ammo??


what ammo are you using when this happens??

Ammo is WWB, I have had this problem with several known good 30 rd mags as well as partially loaded 30 rders. I will let you know what a 20 rder does in about 10 minutes

Pull the buffer and spring  and check the spring for end of coils bur (carrier side).  If needed, use a file to remove the burs. Also check the roll pin on the buffer bumper to confirn that it is not sticking out and binding on the spring.  
Don't forget to apply a little lube to the extension, buffer, and the spring before reinstalling.

Second idea is that the recoil spring may just be on the weak side from the start.

Last idea is that the hammer is slightly catching on the firing pin, and delay could be the problem.

My 20 round mag went off without a hitch tonight, I will stretch the buffer spring and check some of the other stuff you mentioned on my next visit to the range Thanks

Quoted: How many rounds are you stuffing into the mag?? FWIW I ,as a rule, only put 28 rounds in my mags. does this happen across several known good magazines?? tried different ammo?? what ammo are you using when this happens??

Sounds like loading one less round in your 30's will cure your problem, though the other suggestions are good ones too. Strong mag springs are good but it can be overdone.

I had to change buffer springs to wolff xp on both of my builds to get the rnds to chamber properly. Especially the first few off a fully loaded 30 rnder. There was quite a bit of difference in the strength of the 2 original springs with one being very week. Just make sure you check for any mechenical problems before you use an xp spring.