Multiaccess communications, governed by protocol signals, over a slot-synchronized collision channel without feedback, is considered. More specifically, we consider the channel model proposed by Massey and Mathys, augmented by interleavers that are essential for enabling the usage of practical short codes. We assess the performance on the basis of a (so called) shortcoding error parameter. The set of the parameters of all users characterizes the rate region achievable by using short block codes with nonideal interleaving. The short-coding error parameter corresponds to a bound on the probability of error which is tighter than the bound obtained by using the conventional cutoff rate. The analysis brings into account practical communication aspects, such as the effect of interleaving span and background noise in noninterfered slots.