A Bullpup Avenger?

Point is, the top half is the cooling section/mag(clip)well and might have an alternative use (how do Soldiers fire Concussive Shot etc. exactly?) and the entire point of a bullpup is to reduce rifle length so you can have a long barrel in that shorter package.

For example (all pulled from Wiki, don't need a better source for this), the TAR-21 has an overall length of 720mm with a barrel length of 460mm. M4 Carbine, with stock retracted (minimizing the length, as you can't take the buffer tube off the M4, since that's where the bolt goes) is 756mm long (LONGER than the TAR-21) with a barrel of 370mm. This means that the M4 is LONGER but has a SHORTER BARREL compared to the bullpup TAR-21.

You might argue that the M4 is a CARBINE while the TAR-21 is an ASSAULT RIFLE, but the CTAR-21 (Carbine variant) is still 640mm long with a 380mm barrel. That means barrel is comparable length while the CTAR-21 is full twelve centimeters shorter. For the record, a longer barrel gives you a greater muzzle velocity and higher precision.

In Mass Effect, firearms work with an element zero core that accelerates rounds to great velocities (Avenger muzzle velocity is never stated, though, as is the case with all other weapons in universe) referred to as "relativistic". That's "fucking fast" to translate, and modern railguns being tested by the US Military can reportedly reach a 2.5km/s (TWO AND A HALF KILOMETER PER SECOND!) muzzle velocity. Basically you're only working with accuracy, and the Avenger is likely precise to extreme ranges compared to today's assault rifles (300m or thereabouts of point accuracy).

You're basically working with small-arms sized railguns here. Railguns could easily have an effective range of many kilometers, even as direct fire weapons (not artillery - I shudder to imagine the range then, and the projectiles would probably pass through the stratosphere on high angle shots).

Do you really need a bullpup Avenger? I'd rather see an adjustable folding stock (like the FN SCAR/Mk16/Mk17 has) on it.
I've actually taken a look at the Avenger, and bullpuping it would probably increase the minimal length of the rifle, as right now, you could probably take off the stock right where the black portion past the pistol grip ends and not significantly influence function.

Which is what the US armed forces use at ranges to rate your marksmanship skills. You have to hit at least 2 "Ivans" (that's what they were called then. I understand it's changed.) at 300m to score Expert. I got mine at Ft. Drum because I needed the points to get into PLDC class. Also, at the time, there were no attachments allowed. It was just iron sights that you zeroed using a clip supplied. Hitting them with iron sights at 300m (and with a stand-up time of about three to five seconds) was difficult. I attempted the range twice before getting 39 out of 40. Before that, 33 because those 275 and 300 targets were a bitch. I had to re-zero the "clankety" mess one more time because the range instructor was generous.