これを試して;
private const int PostsPerFeed = 25; //Change this to whatever number you want
次に、あなたの行動:
    public ActionResult Rss()
    {
        IEnumerable<SyndicationItem> posts =
            (from post in model.Posts
             where post.PostDate < DateTime.Now
             orderby post.PostDate descending
             select post).Take(PostsPerFeed).ToList().Select(x => GetSyndicationItem(x));
        SyndicationFeed feed = new SyndicationFeed("John Doh", "John Doh", new Uri("http://localhost"), posts);
        Rss20FeedFormatter formattedFeed = new Rss20FeedFormatter(feed);
        return new FeedResult(formattedFeed);
    }
    private SyndicationItem GetSyndicationItem(Post post)
    {
        return new SyndicationItem(post.Title, post.Body, new Uri("http://localhost/posts/details/" + post.PostId));
    }
FeedResult.csで
class FeedResult : ActionResult
{
    private SyndicationFeedFormatter formattedFeed;
    public FeedResult(SyndicationFeedFormatter formattedFeed)
    {
        this.formattedFeed = formattedFeed;
    }
    public override void ExecuteResult(ControllerContext context)
    {
        context.HttpContext.Response.ContentType = "application/rss+xml";
        using (XmlWriter writer = XmlWriter.Create(context.HttpContext.Response.Output))
        {
            formattedFeed.WriteTo(writer);
        }
    }
}
デモストレーションはこちらです。警告ですが、グーグルクロームのフォーマットはまだありません